CHEM 0310 Ch 2
What does ΔS mean in chemical reactions?
"Disorder" is not readily quantifiable. Chemically, ΔS° is used to describe changes in energy dispersal. Going from lesser to greater energy dispersal is thermodynamically favorable.
carbonyl functional group
- (C＝O) - found in aldehydes, ketones, and in conjunction with an attached 一OH in carboxylic acids
bomb calorimetry
- Comparing amount of heat given off through combustion between compounds on a "/g" basis will allow for a ranking of thermodynamic stabilities for the compounds provided that the products are formed in an identical ratio because products have equal energy while reactants have different energy - Less stable compounds undergo combustion to give more heat - In general, compounds that yield a higher H2O/CO2 product ratio give off more energy on a per gram basis because of the extremely strong oxygen-hydrogen bonds in water - This analysis is the reason for enthusiasm about natural gas (methane), though fuel cells that generate and burn H2 have even greater potential for environmental benign energy production
Synthesis
- Converting inexpensive / non-complex molecules to valuable materials - Know our chemical reactions - Develop a strategy for sequencing reactions
Mechanism
- Detailed steps of chemical reactions - Thermodynamics - Will a reaction occur? - Kinetics - How fast will a reaction occur?
Structure
- Shapes of molecules - Tools of structural determination
ETHANE - Dihedral angle vs. Energy
- Staggered conformation is favored (lower in energy due to the avoidance of steric interactions) - Eclipsed conformations are energy maxima - The transition state is not isolable - Rotational barrier = 2.9 kcal/mol for ethane
origin of torsional strain in eclipsed conformations
- brings pairs of C-H bonds closer, repulsion between electrons increases - C -- C bonds weaken due to changes in orbital interactions
common structural feature of all alkanes
- carbon chain - this chain influences the physical properties of not only alkanes but also any organic molecules possessing such a backbone
enthalpy change of a reaction (ΔH°)
- heat absorbed or released at constant pressure during the reaction - measure of molecular stability - differences between the strengths of the bonds in the products compared with those in the starting materials
alkanes
- lack functional groups, so they are relatively nonpolar and unreactive - Hydrocarbons containing only single bonds (methane, ethane, propane) are
three-dimensional structures of alkanes
- make use of the hashed-wedged line notation - main chain and a hydrogen at each end are drawn in the plane of the page
ΔS°
- measure of disorder - Increasing disorder correlates with an increase in the value of S°
Hydrocarbons
- molecules that contain only hydrogen and carbon - have the general empirical formula CxHy
primary carbon
- one attached directly to only one other carbon atom - all carbon atoms at the ends of alkane chains are primary
PENTANE - neopentane
- smallest molecule that can contain a carbon that's bonded to four other carbons - shortest compound that has the potential for conformations that have long range steric clashes called syn-pentane interactions
R represents a part of an alkane molecule
- stands for radical or residue - commonly used to represent an alkyl group
METHANE
-a natural gas (weak dispersion attractions to itself) - energy source. - low molecular weight hydrocarbon - sp3 hybridized and tetrahedral - C一H bonds are sigma bonds
Many functional groups contain polar bonds
Haloalkanes Alcohols Ethers
Le Chatelier's: The pressure might change.
Increasing the pressure on a gas reaction shifts equilibrium towards the side with fewer moles.
Are these factors (thermodynamics and kinetics) directly related?
No. Reactions that are thermodynamically very favorable often proceed faster than less favorable ones. But, some reactions are faster than others even though they result in a comparatively less stable product.
IUPAC Rule 3
Number the carbons of the longest chain beginning with the end that is closest to a substituent. If there are two substituents at equal distance from the two ends of the chain, use the alphabet to decide how to number. The substituent to come first in alphabetical order is attached to the carbon with the lower number.
stem chain
the longest chain
The rate constant equals
the rate of the reaction at 1 molar concentrations of the *reactants*
neo-
used when all but two carbons form a continuous chain, and these two carbons are part of a terminal tert-butyl group
iso-
used when all carbons except one form a continuous chain
n- (normal)
used when all carbons form a continuous, unbranched (linear) chain
tert- (or t-)
used when the functional group is bonded to a tertiary carbon
Naming haloalkanes
To name haloalkanes, treat the halogen as a substituent to the alkane framework. This is no different! The longest stem chain is numbered so the first substituent from either end receives the lowest number. Substituents are ordered alphabetically, and complex appendages are named using the rules for complex alkyl groups.
Heat of combustion
amount of heat a compound produces, measured through calorimetry
primary alkyl
an alkyl group created by removing a primary hydrogen
Dihedral angle
angle between two bonds that are separated by one bond
ETHANE - Dihedral angle
angle between two bonds that are separated by one bond
How do you draw a newman projection?
We turn the molecule out of the plane of the page toward us and view it along the C - C axis. It gives an end on view that shows relative orientation of substituents on adjacent atoms. The front carbon obscures the back carbon, but the bonds emerging from both are clearly seen.
METHANE - Homolytic bond dissociation
produces 2 of the same species (radicals) For methane, its BDE is 105 kcal/mol, which means it's a very strong bond
reaction rate
product of the rate constant of the conversion of intermediates over the highest transition state (rate determining step) and concentration of all species that react in this step
Radical Halogenation
rate = k [R--H][X .]
alcohol general formula
represented as R - O - H
activation energy
the energy input required to raise the energy of the starting compounds to that of the transition state a barrier to the reaction; the difference in energy between a starting material and a transition state
PENTANE (C5H12)
C5H12 compounds are the smallest hydrocarbons to be liquid at room temperature as a result of VDW dispersion forces (transient dipoles)
methylene group
CH2
methyl group
CH3
What three factors cause an increasing rate of chemical reactions?
1. Increasing temperature 2. Decreasing activation energy 3. Increasing concentration
Disubstitution
A disubstituted alkene is an alkene in which the doubly bonded carbons are bonded to a total of two carbon atoms
PENTANE - neopentyl substituent
A functional group that is bonded to a carbon that is adjacent to a quaternary carbon
symmetrical ether
A general formula for an ether in which both alkyl groups are the same (symmetrical ether) is R - O - R
Monosubstitution
A molecule or functional group in which only one hydrogen has been replaced by another atom or group
What kinds of relationships do we get from Gibbs' equation?
A negative ΔG° signifies a release of energy (a favorable reaction). A large value for K indicates a large favorable free energy change. Because the relation is logarithmic, changing the ΔG° value affects the K value exponentially.
endothermic
A positive ΔH° is characteristic of an endothermic (heat-absorbing) process.
Conformers and abundance
A potential-energy diagram summarizes the energetics of the rotation. The most stable anti-conformer is the most abundant in solution (about 72% at 25 °C). Its less stable gauche counterpart is present in lower concentration (28%).
second order reaction
A reaction where the rate depends on the concentrations of two molecules in this way is said to be a second order reaction.
benzene
A special hydrocarbon in which three double bonds are incorporated into a six-membered ring (every other bond is a double bond). This compound and its derivatives are aromatic (called arenes) because some substituted benzenes have a strong fragrance.
pattern of alkanes
Alkane chains often adopt the zigzag patterns used in bond-line notation.
amines
Alkyl-nitrogen compounds are amines.
unsymmetrical ether
An ether with two dissimilar groups (unsymmetrical ether) is represented by R - O - R'.
What happens before, during, and after atoms pass through the transition state?
As atoms move from their initial positions, energy input is required for bond breaking. At the transition state, where both partially broken old bonds and incompletely formed new ones are present, the overall loss of bonding reaches its greatest extent, and the energy content of the system is at its maximum. Beyond this point, continued strengthening of the new bonds releases energy until the atoms reach their final, fully bonded positions in the products.
The regularity in alkane structures suggests that their physical constants would follow predictable trends
At room temperature (25°C), the lower homologs of the alkanes are gases or colorless liquids while the higher homologs are waxy solids. From pentane to pentadecane, each additional CH2 group causes a 20 - 30°C increase in boiling point.
skew conformations
Between the two extremes (staggered and eclipsed), rotation of the methyl group results in numerous additional positions, referred to collectively as skew conformations.
Curved arrows show how starting materials convert to products
Bonds consist of electrons. Chemical changes result in bonds being broken or formed. When chemistry takes place, electrons move. The description of this electron movement constitutes a reaction mechanism. A curved arrow will show the flow of an electron pair from its point of origin (lone pair or covalent bond) to its destination. The target may be an atom that attracts the electrons by virtue of being relatively electronegative or electron deficient.
Branched alkanes are constitutional isomers of straight-chain alkanes
Branched alkanes are derived from the straight-chain systems by removal of a hydrogen from a methylene group and replacement with an alkyl group. Both branched and straight-chain alkanes have the same general formula, CnH2n+2 .
Branched Alkanes: Melting and Boiling Points
Branched alkanes have smaller surface areas than do their straight-chain isomers. They are generally subject to smaller London attractions and are unable to pack as well in the crystalline state. The weaker attractions result in lower melting and boiling points. Branched molecules with highly compact shapes are exceptions. Crystal packing differences account for the slightly lower than expected melting points of odd-membered straight-chain alkanes relative to those of even-membered systems.
BUTANE (n-butane)
C4H10
common or trivial names
Compounds have been named after their discoverers, after localities, shapes, and natural sources. Many of these common or trivial names are still widely used.
There can be more than one staggered and one eclipsed conformation: conformational analysis of butane
Consider the staggered conformer in which the two methyl groups are as far away from each other as possible. This arrangement (anti) is the most stable because steric hindrance is minimized. Rotation of the rear carbon in the Newman projections clockwise produces an eclipsed conformation with two CH3 - H interactions. This conformer is 3.6 kcal/mol higher in energy than the anti-precursor. Further rotation furnishes a new staggered structure in which the two methyl groups are closer than they are in the anti-conformation (gauche conformation). As a consequence of steric hindrance, the gauche conformer is higher in energy than the anti-conformer by 0.9 kcal/mol. Further rotation results in a new eclipsed arrangement in which the two methyl groups are superposed. Because the two bulkiest substituents eclipse in this conformer, it is energetically highest, 4.9 kcal/mol higher than the most stable anti-structure. Further rotation produces another gauche conformer. The activation energy for gauche ⇌ gauche interconversion is 4.0 kcal/mol.
kinetic control
Describes a reaction in which the product obtained is the one formed the fastest
thermodynamic control
Describes a transformation that yields the most stable products Its outcome is determined by the net favorable change in energy (decrease in energy) in going from starting materials to products.
PROPANE
Differs from ethane by the presence of a methylene (CH2) group
alkenes and alkynes
Double and triple bonds are the functional groups of alkenes and alkynes, respectively.
Consider the combustion of methane; this releases a huge amount of energy, but methane does not spontaneously ignite in air at room temperature. Why is this highly favorable combustion process is so slow?
During the course of this reaction, the PE of the system changes. The transition state for methane combustion is very high in energy, which corresponds to a high Ea and a very low rate.
What is carbon bound to in straight-chain alkanes?
Each carbon is bound to its two neighbors and to two hydrogen atoms. Exceptions are the two terminal carbon nuclei, which are bound to only one carbon atom and three hydrogen atoms.
What is the dihedral angle for the staggered and eclipsed conformations of ethane?
Eclipsed → 0 degrees for HCCH Staggered → 60 degrees for HCCH
IUPAC Rule 1
Find the longest chain in the molecule and name it. In the condensed formula, complex alkanes may be written in ways that mask the identity of the longest chain. Do not assume that it is always depicted horizontally! The alkane stem gives the molecule its name. If a molecule has two or more chains of equal length, the chain with the largest number of substituents is the base stem chain.
Consider a transformation in which the number of reacting molecules differs from the number of product molecules formed.
For example, upon strong heating, 1-pentene undergoes cleavage into ethene and propene. This process is exothermic, primarily because a C - C bond is lost.
IUPAC Rules (in summary)
Four rules should be applied in sequence when naming a branched alkane: (1) Find the longest chain (2) find the names of all the alkyl groups attached to the stem (3) number the chain (4) name the alkane, with substituent names in alphabetical order and preceded by numbers to indicate their locations
IUPAC Naming Rules
IUPAC nomenclature provides a systematic method for naming organic molecules
Value of K
If K > 1, the mixture contains mostly products. A large K (Keq) indicates a reaction goes to completion due to a large driving force. If K < 1, the mixture contains mostly reactants. If K = 1, the mixture is in equilibrium.
Le Chatelier's Principle
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
IUPAC Rule 4: Complex substituents
If a particular complex substituent is present more than once, a special set of prefixes is placed in front of the parenthesis: bis, tris, tetrakis, pentakis, and so on. In the chain of a complex substituent, the carbon numbered one (C1) is always the carbon atom directly attached to the stem chain.
When does Gibbs standard free energy change come into play?
If a reaction has gone to completion, a certain amount of energy has been released.
exothermic reaction
If the bonds formed are stronger than those broken, ΔH° is negative and the reaction is exothermic (releasing heat)
There are two extreme ways of drawing ethane: the staggered conformation and the eclipsed one.
If the staggered conformation is viewed along the C - C axis, each hydrogen atom on the first carbon is seen to be positioned perfectly between two hydrogen atoms on the second. The second extreme is derived from the first by a 60° turn of one of the methyl groups about the C - C bond. If this eclipsed conformation is viewed along the C - C axis, all hydrogen atoms on the first carbon are directly opposite those on the second — that is, those on the first eclipse those on the second. A further 60° turn converts the eclipsed form into a new but equivalent staggered arrangement.
IUPAC Rule 3 - What if there are three or more substituents?
If there are three or more substituents, follow the first point of difference principle. Number the chain in the direction that gives the lower number at the first difference between the two possible schemes.
Rotation interconverts the conformations of ethane
If we build a molecular model of ethane, we can see that the two methyl groups are readily rotated with respect to each other. The energy required to move the hydrogen atoms past each other (barrier to rotation) is 2.9 kcal/mol. This value is so low that chemists speak of "free rotation" of the methyl groups. In general, there is free rotation about all single bonds at room temperature.
on which side of a reaction does the equilibrium lie?
In many cases, equilibrium lies extensively (say, more than 99.9%) on the product side. When this occurs, the reaction is said to go to completion. In such cases, the arrow indicating the reverse reaction is omitted, and the reaction is considered to be irreversible.
When can we approximate ΔG° = ΔH°?
In many organic reactions the change in entropy is so small that it often suffices to consider only the changes in bonding energy to estimate whether they are likely to occur or not.
first order reaction
In some processes, the rate depends on the concentration of one reactant
What if there is more than one alkyl group? How do you distinguish one R from another?
In structures that contain multiple alkyl groups, we add prime (') or double prime ('') to R to distinguish groups that differ in structure.
What's the significance of having a rotational barrier < 20 kcal/mol?
It leads to rapid processes at room temperature
ETHANE
Its physical properties are similar to those of methane since it's essentially 2 methyl groups bonded together
Difference in thermodynamic stability of two conformers
Knowing the difference in thermodynamic stability of two conformers (e.g., 0.9 kcal/mol between the anti and gauche isomers) and the activation energy for proceeding from the first to the second (e.g., 3.6 kcal/mol) allows us to estimate the activation barrier of the reverse reaction. In this case, Ea for the gauche to anti conversion is 3.6 - 0.9 = 2.7 kcal/mol.
Strength (or lack thereof) of London forces
London forces are very weak. London forces fall off as the sixth power of the distance between molecules. There is also a limit to how close these forces can bring molecules together. At small distances, nucleus-nucleus and electron-electron repulsions outweigh these attractions. These forces account for the physical constants of elements and compounds. To cause melting, the attractive forces responsible for the crystalline state must be overcome.
The nonpolar alkanes attract each other by
London-Dispersion forces, which are due to electron correlation When one alkane molecule approaches another, repulsion of the electrons in one molecule by those in the other results in correlation of their movement. Electron motion causes temporary bond polarization in one molecule, and correlated electron motion in the bonds of the other induces polarization in the opposite direction, resulting in attraction.
We will learn about the following things in this course
Mechanism Structure Synthesis
Alkanes: Melting and Boiling Point
Melting points rise with increasing molecular size: Molecules with relatively large surface areas are subject to greater London attractions. These forces are still relatively weak, and even high-molecular-weight alkanes have rather low melting points. For a molecule to escape these same attractive forces in the liquid state and enter the gas phase, more heat has to be applied. Boiling points of compounds are also relatively high if the intermolecular forces are relatively large. These effects lead to the smooth increase in boiling points.
Collisions supply the energy to get past the activation-energy barrier
Molecules have KE as a result of their motion, but at room temperature the average KE is only about 0.6 kcal/mol, which is far below many activation-energy barriers. To pick up enough energy, molecules must collide with each other or the walls of the container.
cycloalkanes
Molecules whose carbons form a ring (with carbon carbon single bonds)
IUPAC Rule 2
Name all groups attached to the longest chain as alkyl substituents. For straight-chain substituents, Table 2-5 can be used to derive the alkyl name. In this case, the same IUPAC rules apply to complex substituents: First, find the longest chain in the substituent; next, name all of its substituents.
PROPANE - Rotational Barrier
Propane's rotational barrier is significantly higher than ethane's (2.9) due to increased steric interaction between the methyl and hydrogen group in the eclipsing conformation.
haloalkane general formula
R - X where R stands for any alkyl group and X for any halogen
IUPAC Rule 4 - What if a molecule contains more than one of a particular substituent?
Should a molecule contain more than one of a particular substituent, its name is preceded by the prefix di, tri, tetra, penta, and so forth. The positions of attachment to the stem are given collectively before the substituent name and are separated by commas. These prefixes, as well as sec- and tert-, are not considered in the alphabetical ordering, except when they are part of a complex substituent name.
Steric hindrance raises the energy barrier to rotation
The Newman projections of propane differ from those of ethane only by the substituted methyl group. The extreme conformations are staggered and eclipsed (still), but the activation barrier separating the two is 3.2 kcal/mol, slightly higher than that for ethane. This is due to unfavorable interference between the methyl substituent and the eclipsing hydrogen in the transition state (steric hindrance). This effect arises from the fact that two atoms or groups of atoms cannot occupy the same region in space. Steric hindrance in propane is worse than the Ea value for rotation indicates. Methyl substitution raises the energy not only of the eclipsed conformation, but also of the staggered (lowest-energy, or ground state) conformation, the staggered to a lesser extent because of less hindrance. However, because the activation energy is equal to the difference in energy between ground and transition states, the net result is only a small increase in Ea.
Alkanes are very useful compounds
The branched alkane 2,2,4-trimethylpentane is an important component of gasoline - the standard on which the "octane rating" system for fuel efficiency is based. Racecars require high-octane fuel.
Structural features of the alkanes are remarkably regular
The carbon atoms are tetrahedral, with bond angles close to 109° and with regular C - H (approx 1.10 Å) and C - C (approx 1.54 Å) bond lengths.
rotational (torsional) strain
The change in energy resulting from bond rotation from the staggered to the eclipsed conformation
ethers
The characteristic functional group of ethers 一 oxygen bonded to two carbon atoms
Naming the alkanes - The Problem
The diversity that exists causes a problem. Compounds have been named after their discoverers, after localities, shapes, and natural sources. Many of these common or trivial names are still widely used.
understanding a PE diagram
The energy first rises to a maximum (transition state) before decreasing to the final value, which is the energy content of the product molecules. The energy of the transition state is a barrier to be overcome.
Gibbs standard free energy change - equation
The equilibrium constant (K) can be related directly to Gibbs standard free energy change.
Exothermic Reaction EXAMPLE: Combustion of methane (main component of gas) to CO2 & H2O
The exothermic nature is due to the very strong bonds formed in the products. Many hydrocarbons release a lot of energy during combustion and are valuable fuels.
The five common group names permitted by IUPAC
The five common group names permitted by IUPAC are the following: isopropyl, isobutyl, sec-butyl, tert-butyl, and neopentyl. These five are used universally in the course of normal communication between scientists. Even yet, it is still preferable to use systematic names, especially when searching for information. The systematic name should be enclosed in parentheses to avoid ambiguities.
Newman projection skeleton
The front carbon is depicted as the point of juncture of the three bonds attached to it, one of them usually drawn vertically and pointing up. The back carbon is represented by a circle. The bonds to this carbon project from the outer edge of the circle.
primary hydrogen
The hydrogens attached to primary carbons are
Le Chatelier's: The temperature might change.
The position of equilibrium moves to the left if you increase temperature in an exothermic reaction.
rate determining step
The rate-determining step since is the one that has the highest energy.
thiols
The replacement of oxygen in alcohols by sulfur furnishes thiols
BUTANE - rotational energy profile
The rotational energy profile for the C2-C3 bond of n-butane is more complicated than that for propane. Instead, we will look at the profile for t-butyl X.
secondary, tertiary, and quaternary structures
The same logic applies to secondary, tertiary, and quaternary carbons, hydrogens, and alkyl groups. A carbon bearing four alkyl groups is quaternary. The terms primary, secondary, tertiary, and quaternary are reserved for carbon atoms with exclusively single bonds.
Boltzmann distribution curve - graph
The shape of the curve depends on the temperature. At higher T, as the average kinetic energy increases, the curve flattens and shifts toward higher energies. More molecules now have energy higher than is required by the transition state, so the speed of the reaction increases. At lower temperatures, the reaction rate decreases.
alkanes as a backbone
The skeleton of most organic molecules is a relatively nonpolar assembly consisting of carbon and hydrogen atoms connected by single bonds.
2-methylpropane (isobutane)
The smallest branched alkane is 2-methylpropane. It has the same molecular formula as butane (C4H10) but different connectivity. The compounds are, therefore, constitutional isomers.
The conformers of ethane have different potential energies
The staggered conformer is the most stable and lowest energy state of the molecule. The potential energy rises as the structure moves away from the staggered geometry, through skewed shapes, finally reaching the eclipsed conformation. At the point of eclipsing, the molecule has its highest energy content, about 2.9 kcal/mol above the staggered conformation. . As rotation to the eclipsed geometry brings pairs of C - H bonds on the two carbons closer to each other, repulsion between the electrons in these bonds increases. Rotation also causes subtle changes in molecular orbital interactions, weakening the C - C bond in the eclipsed conformation.
Straight-chain alkanes form a homologous series
The straight-chain alkane series may be described by the general formula H-(CH2)n-H. Each member of this series differs from the next lower one by the addition of a methylene group (CH2). Molecules that are related in this way are homologs of each other, and the series is a homologous series.
A potential-energy diagram can be used to picture the energy changes associated with bond rotation.
The x-axis denotes degrees of rotation, usually called torsional angle. The figure sets 0° at the energy minimum of a staggered conformation, the most stable geometry of the ethane molecule. The eclipsed conformer occurs at an energy maximum: Its lifetime is extremely short (less than 10-12 s), and it is a transition state between rapidly equilibrating staggered arrangements. The 2.9 kcal/mol energy difference between the staggered and eclipsed conformations corresponds to the activation energy for the rotational process. All organic molecules with alkane-like backbones exhibit such rotational behavior.
METHANE - Monosubstitution
There is one product (Methyl X or Me X), since all C-H bonds are identical
A system for naming alkanes
There now exists a precise system for naming the alkanes. Systematic nomenclature, in which the name of a compound describes its structure, was first introduced by a chemical congress in Geneva, Switzerland, in 1892. It has continually been revised since then by IUPAC. The stems of the straight-chain alkanes (mainly of Greek origin) reveal the number of carbon atoms in the chain. For example, heptadecane is composed of the words hepta, seven, and deka, ten. The first four alkanes have special names that have been accepted as part of the systematic nomenclature but also all end in -ane. A few smaller branched alkanes have common names that still have widespread use. They make use of the prefixes iso- and neo-.
Boltzmann distribution curve
This depicts the distribution of kinetic energy (KE). Although most molecules have only average speed at any given temperature, some molecules have kinetic energies that are much higher.
branched alkyl groups
This table shows a few branched alkyl groups having common names. Some have the prefixes sec- (or s-), which stands for secondary and tert- (or t-), for tertiary. These prefixes are used to classify sp3-hybridized carbon atoms in organic molecules.
What important thing do we get from this relationship between G and k?
This tells us that each energy difference of -1.4 kcal/mol translates to an order of magnitude difference in the equilibrium constant.
What are the two cases when we can NOT approximate ΔG° = ΔH°?
Transformations in which the number of molecules on each side of the chemical equation differ. Transformations in which energy dispersal is greatly affected by profound structural changes (ring closures and ring openings).
potential energy diagram
Transition state - Fleeting existence - No barrier to change Intermediates - Must pass through a barrier to be converted to something else - A species that is the product of one step of a reaction and the reactant for the next step - High energy intermediates are possible if the whole process is exothermic In this diagram (right), transition state C is the rate-determining step since it has the highest energy.
True or False: Organic chemistry is not a subject where there is hard answers.
True!
True or False: The concentration of reactants can affect reaction rates
True!
PROPANE - Substitution
Two monosubstitution products of propane are possible the substitution of a primary hydrogen (from a methyl) to form an n-propyl compound the substitution of a secondary hydrogen (from a methylene) to form an isopropyl compound (i-Pr).
sec- (or s-)
Used when the functional group is bonded to a secondary carbon This prefix is only useful for a four-carbon chain. It is not applicable with a shorter chain, and it is often ambiguous when the chain has five or more carbons.
IUPAC Rule 4
Write the name of the alkane by first arranging all the substituents in alphabetical order (each preceded by the carbon number to which it is attached and a hyphen) and then adding the name of the stem.
Bond dissociation energy (BDE)
a measure of the strength of a bond; standard enthalpy change when a bond is cleaved by homolysis
bond-dissociation energies, DH°
a quantitative way to describe bond strengths
Higher Ea corresponds to
a slower process
other intermolecular forces
act between molecules
BUTANE - isobutane (2-methylpropane)
branched C4H10 isomer
functional groups
certain structural combinations of atoms in organic molecules that display characteristic and predictable behavior
alkyl halides (aka haloalkanes)
contain polar carbon-halogen bonds as their functional groups
Alcohols
contain the hydroxy functional group; 一O一H
Arrhenius equation
describes how temperature affects reaction rates
arrhenius eqn
e^-Ea/RT - fraction of molecules with E > Ea A - frequency of collisions k is smaller with increasing Ea values and higher as T increases Raising the reaction temperature by 10 ℃ causes the rate to increase by a factor of 2 to 3. Note the units: R is in units of 8.31 J/kmol Ea is in units if J/mol
Newman projection
end-on view of a bond that is useful for showing relative orientations of substituents on adjacent atoms
Rotational barrier
energetic difference between staggered and eclipsed conformations
reaction coordinate
essentially the progress of the reaction describes the combined processes of bond breaking and bond formation that constitute the overall change from the structures of the starting compounds into those of the products
alkyl group
formed by the removal of a hydrogen from an alkane. It is named by replacing the ending -ane in the corresponding alkane with -yl e.g. methane = CH4 while methyl = CH4 - 1 H = CH3
substituents
groups other than hydrogen attached to the stem chain
Most solid substances exist as
highly ordered crystals Ionic compounds, such as salts, are rigidly held in a crystal lattice, mainly by strong Coulomb forces.
physical properties of alkanes are so predictable because of trends that arise due to
intermolecular or van der Waals forces
When A + B --> C, the rate law is
k is related to the activation energy [A] and [B] are the concentrations of the relevant species in the transition state
conformations (or conformers)
many forms of ethane (and, as we shall see, substituted analogs) are created by such rotations All of them rapidly interconvert at RT
METHANE - Combustion
most common reaction for methane (and other hydrocarbons)
Monosubstitution
one hydrogen is replaced by another atom or group
potential energy diagram definition
plots energy as a function of reaction progress
conformational analysis
the study of the thermodynamic and kinetic behavior of different conformations
Nonionic but polar molecules, such as chloromethane (CH3Cl), are attracted by
weaker dipole - dipole interactions, also of coulombic origin
equilibrium
when the concentrations of reactants and products no longer change @ this point, K = 1
How do you estimate ΔH?
ΔH° for a reaction may be estimated by subtracting the sum of DH° values of bonds formed from those of bonds broken
