Orgo Quizlet Three
cis-trans isomers
carbons have covalent bonds to the same atoms, but these atoms differ in their spatial arrangements due to the inflexibility of double bonds
The chirality center in 2-butanol is a _____stereogenic center.
tetrahedral
steric strain
the interference between two bulky groups that are so close together that their electron clouds experience a repulsion
CIS AND TRANS isomers are also STEREOISOMERS becaus
they differ from each other only in the arrangement of their atoms in space
Enantiomers show different behavior only when they
they interact with other chiral substances, including their own enantiomer.
when you have a molecule where the configuratin of each chiral center changes,
those two molecules are known enantiomers
Each triple bond consumes
two molar equivalents of hydrogen
chiral molecules
-a molecule that is not super-imposable upon its mirror image or lacks an internal plane of symmetry
SN2 reaction
-bimolecular nucleophilic substitution reactions - only 1 step (concerted reaction) -nucleophile attacks the compound at the same time as the leaving group leaves -Nucleophile actively displaces the leaving group in a backside attack for this to occur, nucleophile must be strong & substrate can't be sterically hindered -concentrations of substrate & nucleophile have role in determining the rate --> rate = k[Nu][R-L] -Position of the substituents around the substrate carbon is inverted
each double bond or ring provides ____ unit of hydrogen deficiency
1
sn2 reactions have ___ transition state
1
Both 1-hexene and cyclohexane (C6H12) have an index of hydrogen deficiency equal to
1 (meaning one pair of hydrogen atoms)
In ethane the energy difference between the staggered and eclipsed conformations is about
12 KJ MOL-1
SEC-BUTYL ALCOHOL
2-butanol
number of possible stereoisomers
2^n
cholesterol
8 chirality center
R,S system
A METHOD FOR DESIGNATING THE CONFIGURATION OF TETRAHEDRAL CHIRALITY CENTERSa set of rules for specifying the configuration about a stereocenter
nucleophilic substitution reactions
In Sn1 & Sn2 reactions, a nucleophile forms a bond with a substrate carbon and a leaving group leaves
torsional strain
Increased energy that results when molecules assume eclipsed or gauche staggered conformations. caused by repulsion between aligned electron pairs of the eclipsed bonds
lipase
Lipase catalyzes a reaction called hydrolysis, whereby an ester (Section 2.10B) reacts with a molecule of water to produce a carboxylic acid and an alcohol.
Studies with other substituents show that there is generally less repulsion when any group larger than hydrogen is
equatorial rather than axial.
If a reaction takes place in a series of steps, and if one step is intrinsically slower than all the others, then the rate of the overall reaction will be
essentially the same as the rate of this slow step.
in cyclohexane derivatives with larger alkyl substituents, the strain caused by 1,3-diaxial interactions is
even more pronounced.
A molecule will not be chiral if it
possesses a plane of symmetry (an imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other.
Conformations of cyclohexane rings
planar structurw boat twist boat chair conformation
if two molecules are not superimposable
they are DIFFERENT COMPOUNDSd
acetylene
C2H2 aka ethyne
polar protic solvents
*increase the rate of SN1 and decrease the rate of SN2. CAN HYDROGEN BOND Polar protic solvents are water, ethanol, methanol, ammonia, acetic acid, and others. Polar aprotic solvents contain no hydrogen atoms connected directly to an electronegative atom and they are not capable of hydrogen bonding.
SN1 reaction
*unimolecular nucleophilic substitution reactions: 2 steps 1. Leaving group leaves forming a positively charged carbocation (rate limiting step) *The rate of rxn depends only on the concentration of the substrate rate = k [R-L] R-L is the alkyl group containing the leaving group *Anything that accelerates the formation of the carbocation increase the rate of rxn 2. Nucleophile attacks the carbocation (unstable) *results in substitution product
Chirality centers are often designated with an
asterisk (*).
what leads to steric hindrance
when dispersion forces are repulsive
Fused rings have ___carbons in their bridge
zero
chirality
A property of a compound to exist in both left and right forms; occurs whenever a compound contains an asymmetric carbon having the property of handedness (the case where mirror images of an object are not superimposable) Each of our hands is chiral
racemization
A reaction that transforms an optically active compound into a racemic form is said to proceed with racemization the loss of optical activity that occurs when a reaction shows neither clean retention of configuration nor clean inversion of configuration
stereogenic carbon
A single tetrahedral carbon with four different groups attached to it. Also called an asymmetric carbon, a sterocenter, or a chirality center. The last usage is preferred.
anti conformation
A type of staggered conformation in which the two largest groups are antiperiplanar to each other; the most energetically favorable conformation does not have torsional strain from steric hindrance becase the groups are starggered and methyl groups (most bulk parts) are far apart
A SCREDRIVER IS
ACHIRAL
leaving group
Atoms or groups that can dissociate from the parent chain to form a stable species after accepting electron pairs. Weak bases tend to be good leaving groups. leaves the electrophile
monocyclic alkynes generally have the formula
C(n)H(2n+4)
The chirality center in 2-butanol is
C2
ethyne
C2H2
vinyl group
CH2=CH-
allyl group
CH2=CHCH2-
saturated acylic alkanes formula
Cn H2n+2
saturated cycloalkanes
CnH2n
isomers
Compounds with the same formula but different structures. diff compounds that have same molec. formula
cyclopropane
Cyclopropane is necessarily planar (flat), with the carbon atoms at the corners of an equilateral triangle. The 60º bond angles are much smaller than the optimum 109.5º angles of a normal tetrahedral carbon atom, and the resulting angle strain dramatically influences the chemical behavior of this cycloalkane. Cyclopropane also suffers substantial eclipsing strain, since all the carbon-carbon bonds are fully eclipsed
CIS AND TRANS ALKENES ARE
DIASTEREOMERS
CIS AND TRANS CYCLOALKANE ISOMERS ARE
DIASTEREOMERS
2-Propanol and its mirror image
DOES NOT have a chirality center and thuse one can superpose onto the other and THUS they are not enantiomers , we would not predict the existence of enantiomeric forms of 2-propanol, and experimentally only one form of 2-propanol has ever been found.
hyperconjugation vs. resonance
Hyperconjugation involves a sigma orbital component, usually a C-C or C-H bond. Resonance involves pi orbitals
superposable means
NOT ENTANTIOMERS
___molar equivalent of hydrogen is required to reduce an alkene to an alkane.
One
Conformations of Cyclohexane
ROTATIONS about the carbon-carbon single bonds of cyclohexane can produce diff coformations which are interconvertible. chair, boat, twist-boat
EQUATORIAL BONDS
SIX BONDS OF CYCLOHEXANE RING THAT LIE GENERALLY AROUND THE EQUATOR OF THE MOLECULE ALTERNATE FROM slightly up to slightly down Bonds on cyclohexane chair parallel to the ring
stereoisomers
Species with the same structural formula but with a different arrangement of the atoms in space atoms connected in the same sequence (the same constitution), but they differ in the arrangement of their atoms in space have same connectivity and therefore are not constitutional isomers stereoisomers are classified further as being either ENANTIOMERS OR DIASTEROMERS
if a molecule is chiral
it is capable in existing as an enantiomer
how to understand how methylcyclohexane has greater stability with an EQUATORIAL methyl group
Studies done with models of the two conformations show that when the methyl group is axial, it is so close to the two axial hydrogens on the same side of the ring (attached to the C3 and C5 atoms) that the dispersion forces between them are repulsive. This type of steric strain, because it arises from an interaction between an axial group on carbon atom 1 and an axial hydrogen on carbon atom 3 (or 5), is called a 1,3-diaxial interaction. •Studies with other substituents show that there is generally less repulsion when any group larger than hydrogen is equatorial rather than axial.
T R F not all molecules with opposite R,S designations are enantiomers
T MESO COMPOUNDS
AXIAL BONDS
THE 6 BONDS OF A CYCLOHEXANE RING THAT ARE PERPENDICULAR TO THE GENERAL PLANE OF THE RING AND THAT ALTERNATE UP AND DOWN AROUND THE RING perpendicular to the plane
carbocations
TRIGONAL PLANAR! positively charged species with a full positive charge on carbon
chair conformation
The chair-shaped conformation of cyclohexane that has no angle strain and has no torsional strain because it is perfectly staggered about all the C-C bonds. It is strain free. all carbon to carbon bond angles are 109.5 and are therby free of angle strain chair conformation is also free of torsional strain ( rWhen viewed along any carbon-carbon bond (viewing the structure from an end, Fig. 4.12), the bonds are seen to be perfectly staggered. Moreover, the hydrogen atoms at opposite corners of the cyclohexane ring are maximally separated.
ION PAIRING EFFECT
The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an SN1 reaction. It is proposed that solvent or other ions in solution may assist in the removal of a leaving group to form a carbocation which reacts in an SN1 fashion; similarly, the leaving group may associate loosely with the cationic intermediate. The association of solvent or an ion with the leaving group effectively blocks one side of the incipient carbocation, while allowing the backside to be attacked by a nucleophile.
TORSIONAL BARRIER
The energy barrier to free rotation about single bonds in a molecule CAUSED BY ELECTRON PAIR REPULSIONE Ethane has an energy difference between staggered and eclipsed conformers because of this torsional barrier
p orbital of a carbocation
The orbital of a carbocation contains no electrons, but it can accept an electron pair when the carbocation undergoes further reaction.
plane of polarization
The plane in which the electric field oscillates
making alkanes usually is carried out by
The reaction is usually carried out by dissolving the alkene or alkyne in a solvent such as ethyl alcohol (C2H5OH), adding the metal catalyst, and then exposing the mixture to hydrogen gas under pressure in a special apparatus.
Stereoisomers can be subdivided into two general categories:
Those that are enantiomers of each other, and those that are diastereomers of each other.
A chiral molecule and its mirror image are called a pair of
enantiomers. The relationship between them is enantiomeric.
superposable
Two objects are superposable if, when one object is placed on top of the other, all parts of each coincide. To be superposable is different than to be superimposable. Any two objects can be superimposed simply by putting one object on top of the other, whether or not all parts coincide. The condition of superposability must be met for two things to be identical.
conformational stereoisomers
Two stereoisomers whose three-dimensional structures differ only due to rotations about single bonds, and they are interconvertible by bond rotations
bridged bicyclic compound
a bicyclic compound in which the rings share two nonadjacent carbons
STEREOCENTERS
a carbon with four different atoms or groups of atoms bonded to it, aka asymmetric carbon or chiral center
Optically inactive
a compound that does not rotate plane-polarized light
gauche conformation
a conformation with a 60 degree dihedral angle between the largest groups close enough to each other that dispersion forces between the two groups are repulsive this repulsion causes the gauche conformation to have more energy than the anti conformation
CHIRAL
a molecule that is not superimposable on its mirror image
twist boat conformation
a nonplanar conformation of a cyclohexane ring that is twisted from and slightly more stable than a boat conformation the twist boat conformation of cyclohexane has a lower energy than the pure boat conformation, but is not as stable as the chair conformation.
An interchange of groups at the alkene carbons of either 1,2-dichloroethene isomer produces a stereoisomer (a molecule with the same connectivity but a different arrangement of atoms in space), but it does not produce
a nonsuperposable mirror image.
An equimolar mixture of two enantiomers is called
a racemic mixture (or racemate or racemic form). A racemic mixture causes no net rotation of plane-polarized light.
chirality center
a tetrahedral carbon atom that is bonded to four different groups
newman projections
a way of drawing the conformations of a molecule by looking straight down the bond connecting two carbon atoms
All molecules with a plane of symmetry in their most symmetric conformation are .
achiral
a baseball bat is
achiral
a hammer is
achiral
The cis and trans isomers of 1,2-dichloroethene are both
achiral because each isomer is superposable on its mirror image
Enantiomers do not exist for
achiral molecules.
IF the molecule is superposable on its mirror image. THEN IT IS ______. If it is not, the molecule is ____.
achiral; chiral
Molecules of alkanes have no unshared electrons to offer as sites for attack by ___.
acids
conformation analysis
an analysis of the energy changes that occur as a molecule undergoes rotations about single bonds
stereogenic center
any atom at which an interchange of groups produces a stereoisomer when the exchange of two groups bonded to the same atom produces stereoisomers, the atom is said to be a stereogenic atom
why is 2-bromopropane not enantiomeric but 2-butanol is
because the carbon with the chlorine is not an assymetric carbon (connected to 2 methyl groups and a hydrogen and carbon) but for 2-butanol, the carbon is connected to an -OH, hydroven, methyl group, and ethyl group
cis-1,3-Dimethylcyclohexane has a conformation in which both methyl groups are equatorial and one in which both methyl groups are axial. As we would expect, the conformation with ___- is the more stable one.
both methyl groups equatorial
boat conformation
by partial rotations about the carbon0carbon single bonds of the ring. a nonplanar conformation of a cyclohexane ring in which carbons 1 and 4 of the ring are bent toward each other •The boat conformation has no angle strain, but it does have torsional strain. The boat conformation of cyclohexane is formed by "flipping" one end of the chair form up (or down). This flip requires only rotations about carbon-carbon single bonds. flagpole interaction
Enantiomers occur only with compounds whose molecules are
chiral
a golf club is
chiral
an ear is
chiral
A molecule that contains one chirality center is
chiral and can exist as a pair of enantiomers.
DNA is a
chiral molecule. The double helical form of DNA turns in a right-handed way.
If all of the tetrahedral atoms in a molecule have two or more groups attached that are the same, the molecule does not have a
chirality center. The molecule is superposable on its mirror image and is an achiral molecule.
most stable cycloalkane
cyclohexane
Index of Hydrogen Deficiency (IHD)
degree of unsaturation the difference in the number of pairs of hydrogen atoms between the compound under study and an acyclic alkane having the same number of carbons.
cis-trans isomers
diastereomers that differ in their stereochemistry at adjacent atoms of a double bond OR ON DIFFERENT ATOMS OF A RING cIS GROUPS: SAME SIDE OF DOUBLE BOND OR RING TRANS GROUPS : ON OPPOSITE SIDES OF DOUBLE BOND OR RINGhave the same covalent bonds but differ in spatial arrangements
Diastereomers have _____ physical properties—______ melting points and boiling points, _____solubilities, and so forth.
different
conformations
different arrangements of atoms that are interconverted by rotation about single bonds molecular shapes that result from rotations about single bond single: conformer
The central carbon atom in a carbocation is
electron deficient; it has only six electrons in its valence shell
angle strain in cyclopropane
exists because the sp3 orbitals comprising the C-C alpha bonds cannot overlap as effectively as they do in alkanes (where perfect end-on overlap is possible). The carbon-carbon bonds of cyclopropane are often described as being "bent." Orbital overlap is less effective. (The orbitals used for these bonds are not purely sp3; they contain more p character.) The carbon-carbon bonds of cyclopropane are weaker, and as a result the molecule has greater potential energy.
t or f all types of carbocations have the same relative stability
f
Cyclobutane reduces some bond-eclipsing strain by
folding (the out-of-plane dihedral angle is about 25º), but the total eclipsing and angle strain remains high.
eclipsed conformations represent
has energy maxima in the PE diagram eclipsed conformation IV for butane has the greatest energy of all because of the addd large repulsive forces between methyl groups
carbocation stability can be explained on the basis of _____.
hyperconjugation
Pure enantiomers have
identical melting and boiling points.
Separate enantiomers rotate the plane of plane-polarized light equal amounts but
in opposite directions.
ethane spends most of its time
in the lowest energy, staggered conformation, or in a conformation very close to being staggered. Many times every second, however, it will acquire enough energy through collisions with other molecules to surmount the torsional barrier and it will rotate through an eclipsed conformation.
An important property of enantiomers with a single chirality center is that
interchanging any two groups at the chirality center converts one enantiomer into the other.
cyclobutane ring strain
internal angles: 88 (deviates 21 degrees from normal tetrahedral bond angle)
•SN2 reactions always occur with
inversion of configuration.
when u have a meso compound
it is an achiral molecule
Because the carbocation formed in the first step of an SN1 reaction has a trigonal planar structure , when it reacts with a nucleophile,
it may do so from either the front side or the back side
Cyclopentane has very little angle strain (the angles of a pentagon are 108º), but
its eclipsing strain would be large (about 10 kcal/mol) if it remained planar. Consequently, the five-membered ring adopts non-planar puckered conformations whenever possible.
the barriers to rotation about the C2-C3 bond in butane is
larger than for rotation about the C-C bond in ethane
ibruprofen
marketed as a racemate (optically inactive) even though only the (S) enantiomer is the active agent the R isomer of ibruprofen has no antiinflammatory action and is slowly converted to the S isomer in the body
Alkenes and alkynes react with hydrogen in the presence of____ to produce alkanes.
metal catalysts such as nickel, palladium, and platinum
When a racemic mixture reacts with a single enantiomer of another compound, a ______ results,
mixture of diastereomers
When one ring substituent group is larger than the other and they cannot both be equatorial, the conformation with the larger group equatorial will be ___
more stable.
•Because of the greater stability of the chair,
more than 99% of the molecules are estimated to be in a chair conformation at any given moment.
Although it is more stable, the chair conformation is
much more rigid than the boat conformation.
A reaction is said to proceed with retention of configuration at a chirality center if
no bonds to the chirality center are broken. This is true even if the R,S designation for the chirality center changes because the relative priorities of groups around it changes as a result of the reaction.
a chiral molecule is
not superimposable on its mirror image
Each double bond consumes
one molar equivalent of hydrogen
Rings are not affected by hydrogenation, but each ring still counts for
one unit of hydrogen deficiency
Separate enantiomers are said to be ________ because of their effect on plane-polarized light.
optically active compounds
meso compounds are always
optically inactive
alkanes react vigorously with ___
oxygen
The orbitals used for bonds in cyclopropane are not purely sp3; they contain more ___character
p
A chiral molecule and its mirror image are called a
pair of enantiomers
Consequently, like cyclobutane, cyclopentane assumes a slightly bent conformation in which one or two of the atoms of the ring are out of the plane of the others (Fig. 4.10b). This
relieves some of the torsional strain.
angle strain
result of deviation from ideal bond angles caused by inherent structural constraints ( such as ring size ) the incr PE of a molec (usually cyclic) caused by deformation of a bond angle away from its lowest energy value the strain that arises when a bond angle is either compressed or expanded compared to its optimal value
When a beam of plane-polarized light passes through an enantiomer, the plane of polarization
rotates.
hydrogenation
rxn in which hydrogen adds to a double or triple bond. hydrogenation is often acomplished through the use of a metal catalyst such as platinum, palladium, rhodium, or ruthenium The process of converting unsaturated fats to saturated fats by adding hydrogen
constitutional isomers
same molecular formula, different connectivity meaning that their atoms are connected in a different order
Because of torsional barrier,
some molecules will wag back and forth with their atoms in staggered or nearly staggered conformations, while others with slightly more energy will rotate through an eclipsed conformation to another staggered conformation
stereoisomers
ssame molecular form and connectivity but different arrangmenets of atoms in 3D space
chirality center is one type of
stereogenic center
Diastereomers
stereoisomers that are not mirror images of each other
Diastereoisomers
stereoisomers that are not mirror images of each other stereoisomers whose molecules are not mirror images of each other. CIS AND TRANS ALKENES ARE DIASTEREOMERS They have two or more stereocenters.
enantiomers
stereoisomers whose molecules are nonsuperposable mirror images of each other
ENANTIOMERS
stereoisomers whose molecules are nonsuperposable mirror images of each other. molecules that are mirror images of each other
torsional strain
strain associated with eclipsed conformation of a molec; caused by repulsion between the aligned e pairs of the eclipsed bond repulsive dispersion forces that cannot be relieved due to resitricted conformation mobility results when cyclic molecules must assume conformations that have eclipsed interactions
All of the possible stereoisomers for a compound can be drawn by
successively interchanging two groups at each chirality center.
AN achiral molecule is
superposable on its mirror image.
t or f Cycloalkanes do not all have the same relative stability.
t
t or f Stereoisomers are not constitutional isomers.
t
Chirality centers are
tetrahedral stereogenic centers.
dihedral angle
the angle between two specified groups in a Newman projection
. If the cyclobutane ring were planar,
the angle strain would be somewhat less (the internal angles would be 90 instead of 88but torsional strain would be considerably larger because all eight c-h bonds would be eclipsed.
eclipsed conformation
the atoms bonded to carbon at each end ofcarbon-carbon bond are directly opposed to oeach other. diherdral angle between them is 0
ring flip
the change in cyclohexane ring (resulting from partial bond rotations) that convert one ring conformation to another a chair-chair ring flip converts any equatorial substituent to an axial and vice versa a conformational change in which one chair conformation is converted into the other
The most stable conformation for a monosubstituted cyclohexane ring (a cyclohexane ring where one carbon atom bears a group other than hydrogen) is
the conformation where the substituent is equatorial.
Whether dispersion interactions lead to attraction or to repulsion depends on
the distance that separates the two groups
ring strain
the increased PE of the cyclic form of a molec when compared to acycic Energy created in a cyclic molecule by angle strain, torsional strain, and nonbonded strain; determines whether a ring is stable enough to stay intact.
The presence of a single chirality center in a molecule guarantees that
the molecule is chiral and that enantiomeric forms are possible.
achiral
the object and its mirror image are identical A molecule that is superimposable on its mirror image simple geometrical objects such as a sphere or a cube are achiral.
The mirror images of the cis isomer are superposable on each other (try rotating one by 180 to see that it is identical to the other), and therefore the cis formulas both represent
the same, achiral molecule.
If the (R) and (S) enantiomers were present in equal amounts,
there would be no net rotation of the plane of polarized light.
Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon-hydrogen bonds of alkanes are only slightly polarized. As a consequence,
they are generally unaffected by most bases.
The carbon atoms of cis- and trans-1,2-dichloroethene are stereogenic centers, but they are ____- stereogenic centers
trigonal NOT CHIRALITY CENTERS
Cis and trans alkene isomers contain
trigonal stereogenic centers.
t or f there are stereogenic centers that are not chirality centers.
true
inversion
turning inward
alkynes are said to be
unsaturated because the c to c triple bond can react to add two molecules of hydrogen to the molecule, yielding an alkane
