Chapter 4: Alkanes and Cycloalkanes

Branched Alkyl Groups with 5 Carbon Atoms

Alkyl groups bearing five carbon atoms can be branched in many more ways.

Branched Alkyl Groups With 4 Carbon Atoms

Alkyl groups bearing four carbon atoms can be branched in three different ways

4th Step for Naming Bicyclic Compounds

Finally, arrange the substituents alphabetically

Branched Alkyl Group With 3 Carbon Atoms

An alkyl group bearing three carbon atoms can only be branched in one way, and it is called an isopropyl group

6th Rule for Assigning Locants

When a substituent appears more than once in a compound, a prefix is used to identify how many times that substituent appears in the compound(di=2, tri=3, tetra=4, penta=5, and hexa=6). For example, the compound in this image would be called 1,1,3-trimethylcylcohexane. A hyphen is used to separate numbers from letters, while commas are used to separate numbers from each other

Change in Enthalpy(ΔH°)

The heat given off during the reaction. For a combustion process, -ΔH° is called the heat of combustion

Alkyl Groups and Rings

When an alkyl group is connected to a ring, the ring is generally treated as the parent. However, this is only true when the ring is comprised of more carbon atoms than the alkyl group.

5th Rule for Assigning Locants

When dealing with cycolalkanes, all the same rules apply

Drawing Constitutional Isomers of Alkanes

When drawing constitutional isomers of an alkane, make sure to avoid drawing the same isomer twice. To avoid drawing the same compound twice, it is helpful to use IUPAC rules to name each compound.

2nd Rule for Assigning Locants

When multiple substituents are present, assign numbers so that the first substituent receives the lower number. In this image, we number the parent chain so that the substituents are 2,5,5 rather than 3,3,6 because we want the first locant to be as low as possible

Going from Staggered to Eclipsed Conformation in Ethane

When the C-C bond is rotated(going from staggered to eclipsed conformation), the favorable overlap is temporarily disrupted, causing an increase in energy. In ethane, this increase amounts to 12 kJ/mol. Since there are three separate eclipsing interactions, it is reasonable to assign 4kJ/mol to each pair of eclipsing H's

Steric Interactions and Chair Conformation Stability

When the substituent is in an axial position, there are steric interactions with the other axial H's on the same side of the ring. The substituent's electron cloud is trying to occupy the same region of space as the H's that are highlighted, causing steric interactions. These interactions are called 1,3-diaxial interactions

Petroleum

A complex mixture of hydrocarbons which includes alkanes, alkenes, alkynes, and aromatics.

Newman Projection

A kind of drawing specially designed for showing the conformation of a molecule. Used to draw and compare conformations. In this image, one carbon is directly in front of the other, and each carbon atom has three H's attached to it. The point at the center of the drawing represents the front carbon atom, while the circle represents the back carbon

Equatorial-Axial Ratios

Because of the 1,3-diaxial interactions experienced by substituents in axial positions. the equilibrium between the two chair conformations will generally favor the conformation with the equatorial substituent. The exact equiliubrium concentrations of the two chair conformations will depend on the size of the substituent. Larger groups experience greater steric interactions, and the equilubrium will more strongly favor the equatorial substituent.

Naming Bicyclic Compounds with Substituents Continuation

Regardless of the position of substituents, the parent must be numbered beginning with the longest path first. The only choice is which bridgehead will be counted as Carbon 1

Conformations

Rotation about C-C single bonds allows a compound to adopt a variety of possible three-dimensional shapes, called conformations. Some conformations are higher in energy, while other are lower in energy

Complex Substituents Common Names

Some complex substituents have common names. These common names are so well entrenched that IUPAC allows them.

IUPAC(pronounced "I-YOU_PACK")

Stands for The International Union of Pure and Applied Chemistry, which is a A group of scientists which developed a system of organic nomenclature called the Geneva rules

Conformation Analysis of Propane and Energy Diagram

The energy diagram of propane is very similar to that of ethane, except that the torsional strain is 14 kJ/mol rather than 12 kJ/mol. Once again, notice that all staggered conformations are degenerate, as are all eclipsed conformations

Eclipsed Conformation

The conformation highest in energy

Staggered Conformation

The conformation lowest in energy

The Three Staggered Conformations of Butane

The conformation with a dihedral angle of 180° is called the anti conformation, and it represents the lowest energy conformation of butane. The other two staggered conformations are 3.8 kJ/mol higher in energy than the anti conformation.

Conformation Analysis of Ethane and Energy Diagram

The difference in energy between staggered and eclipsed conformations of ethane is 12 kJ/mol, as shown in the energy diagram in this image. Notice that all staggered conformations of ethane are degenerate; that is, all of the staggered conformations have the same amount of energy. Similarly, all eclipsed conformations of ethane are degenerate

Torsional Strain

The difference in energy between staggered and eclipsed conformations of ethane. Its cause is thought to be that the staggered conformation possess a favorable interaction between an occupied, bonding MO and an unoccupied, antibonding MO. This interaction lowers the energy of the staggered conformation.

Selecting the Parent Chain

The first step in naming an alkane is to identify the longest chain, called the parent chain. If there is a competition between two chains of equal length, then choose the chain with the greater number of substituents

Substituents

The groups connected to the parent chain

Summary of the Four Discrete Steps Required for Naming Alkane

1. Identify the parent chain: Choose the longest chain. For two chains of equal length, the parent chain should be the chain with the greater number of substituents. 2. Identify and name substituents. 3. Number the parent chain and assign a locant to each substituent. Give the first substituent the lower possible number. If there is a tie, choose the chain in which the second substituent has the lower number. 4. Arrange the substituents alphabetically. Place locants in front of each substituent. For identical substituents, use di, tri, or tetra, which are ignored in alphabetizing

Hydrocarbons

Compounds comprised of just C and H

Bicyclic Compounds

Compounds that contain two fused rings. Can be drawn in many different ways

Chair Conformation

Has no torsional strain. All H's are staggered. None are eclipsed.

Boat Conformation

Has two sources of torsional strain. Many of the H's are eclipsed and the H's on either side of the ring experience steric interactions called flagpole interactions.

Twist Boat

The boat conformation can alleviate some of this torsional strain by twisting, giving a conformation called a twist boat

Conformations of Cyclohexane

Cyclohexane can adopt many conformations. We will explore two conformations: the chair conformation and the boat conformation. In both conformations, the bond angles are fairly close to 109.5°, and therefore, both conformations possess very little angle strain. The significant difference between them when comparing torsional strain

Drawing Axial and Equatorial Substituents

Each carbon atom in cyclohexane ring can bear two substituents. One groups is said to occupy an axial position, which is parallel to a vertical axis passing through the center of the ring. The other groups is said to occupy an equatorial position, which is positioned approximately along the equator of the ring.

1st Step for Naming Bicyclic Compounds

First identify the parent. In this image, we are dealing with a bicyclic system, so we count the total number of carbon atoms comprising both ring system.. There are seven carbon atoms in both rings combined in this image, so the parent must be "bicyclohept". The two bridgeheads are highlighted. Now count the number of carbon atoms along each of the three possible paths that connect the bridgeheads. The shortest path has not carbon atoms in between the bridgeheads; that is, the bridgeheads are connected directly to each other. Therefore the parent is "bicyclo[3.2.0]hept".

Alkane Combustion

For an alkane, combustion describes a reaction in which the alkane reacts with oxygen to produce CO2 and water

Constitutional Isomers of Alkanes Trends

For an alkane, the number of possible constitutional isomers increases with increasing molecular size. The table in this image illustrates this trend

Alkanes(Saturated Hydrocarbons)

Hydrocarbons that lack pi bonds. The names of these compounds usually end with the suffix "-ane", as seen in the examples in this image

Naming Bicyclic Compounds with Substituents

If a substituents is present, the parent must be numbered properly in order to assign the correct locant to the substituent. To number the parent, start at one of the bridgeheads and begin numbering along the longest path, and then go to the second longest path, and finally go along the shortest path.

1st Rule for Assigning Locants

If one substituent is present, it should be assigned the lower possible number. In this image, we place the numbers so that the methyl group is at C2 rather than C6

4th Rule for Assigning Locants

If the previous rule does not break the tie, such as in this image, then the lowest number should be assigned alphabetically

3rd Rule for Assigning Locants

If there is a tie for the first locant, then the second locant should be as low as possible

Drawing Both Chair Conformations

In a ring containing only one substituent, two possible conformations can be drawn: The substituent can be in an axial position or in an equatorial position. These two possibilities represent two different conformations that are in equilibrium with each other. The term "ring flip" is used to describe the conversion of one chair conformation into the other

Assembling the Systematic Name of an Alkane

In order to assemble the systematic name of an alkane, the carbon atoms of the parent chain are numbered, and these numbers are used to identify the location of each substituent. In each case in this image, the location of the methyl group is clearly identified with a number, called a locant

Assigning Correct Locants

In order to assign the correct locant, we must number the parent chain properly, which can be done by following a few rules

7th Rule for Assigning Locants

Once all substituents have been identified and assigned the proper locants, they are placed in alphabetical order. Prefixes(di, tri, tetra, penta, and hexa) are not included as part of the alphabetization scheme. In other words, "dimethyl" is alphabetized as if it started with the letter "m" rather than "d" Similarly, sec and tert are also ignored for alphabetization purposes, however, "iso" is not ignored. In other words, sec-butyl is alphabetized as a "b" while isobutyl is alphabetized as an "i"

Naming Substituents

Once the parent has been identified, the next step is to list all of the substituents.. Substituents are named with the same terminology used for naming parents, only we add the suffix "yl" instead of "ane". These groups are generically called alkyl groups

Gauche Interaction

In the anti conformation, the methyl groups achieve maximum separation from each other. In the two other conformations, the methyl groups are closer to each other. Their electrons clouds are repelling each other(trying to occupy the same region of space), causing an increase in energy of 3.8 kJ/mol. This unfavorable interaction, called a gauche interaction, is a type of steric interaction, and it is different from the concept of torsional strain

Naming Complex Substituents

In this image, the substituent has five carbon atoms, but it cannot simply be called a pentyl group, because it is not a straight-chain alkyl group. In this situation, the following method is employed: Begin by placing numbers on the substituent, going away from the parent chain. Place number on longest straight chain present in the substituent, in this image, there are four carbon atoms. This group is therefore considered to be a butyl group that has one methyl group attached to it at the 2 position. This group is called a (2-methylbutyl) group. Place parentheses around the complex substituent to avoid confusion, as we will soon place number on the parent chain.

Drawing Both Chair Conformations for a Disubstituted Cyclohexane

In this image, the two conformations for this compound are drawn. Notice that the chlorine atom is above the ring(UP) in both chair conformations, and the methyl group is below the ring(DOWN) in both chair conformations. The configuration(UP or DOWN) does not change during a flip ring. It is true that chlorine is axial in one conformation and equatorial in the other, but a ring flip does not change configuration. The chlorine atom must be UP in both conformations. Similarly, the methyl group must be DOWN in both chair conformations

Naming Bicyclic Compounds

In this image, there are three different paths connecting these two bridgeheads. For each path, count the number of carbon atoms, excluding the bridgeheads themselves. In the compound in this image, one path has two carbon atoms, another has two carbon atoms, and the third(shortest path) has only one carbon atom. These three numbers, ordered from largest to smallest, [2.2.1], are then placed in the middle of the parent surrounded by brackets.

Comparing The Stability of Branched vs. Straight-Chain Alkanes

Isomers that are branched alkanes are lower in energy(more stable) than straight-chain alkanes

Systematic Names

Names produced by IUPAC rules

2nd Step for Naming Bicyclic Compounds

Next, identify and name the substituents

Natural Gas

The C1 to C5 hydrocarbons obtained by the refining of petroleum

Kerosene

The C12 to C18 hydrocarbons obtained by the refining of petroleum.

Gasoline

The C5 to C12 hydrocarbons obtained by the refining of petroleum.

Dihedral Angle(Torsional Angle)

The angle between two groups in a Newman projection. This dihedral angle changes as the C-C bond of ethane rotates and can be any value between 0° and 180°. Therefore there are an infinite number of possible conformations, but there are two conformations that require special attention: the lowest energy conformation and the highest energy conformation. Dihedral angle is 60° in this image

Angle Strain

The increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5°.

An Energy Drawing Showing the Conformational Analysis of Cyclohexane

The lowest energy conformations are the two chair conformations, and therefore, cyclohexane will spend the majority of its time in a chair conformation.

1,3-Diaxial Interactions

The numbers "1,3" describe the distance between the substituent and each of the H's. When the chair conformation is drawn in a Newman projection, it becomes clear that most 1.3-diaxial interactions are nothing more than gauche interactions. The presence of 1,3-diaxial interactions causes the chair conformation to be higher in energy when the substituent is an axial position. When the substituent is an equatorial position, these 1,3-diaxial(gauche) interactions are absent

Conformation Analysis

The study of the three-dimensional shapes of molecules.

Nomenclature

The system of naming chemical compounds

Cycloalkanes

The term "cyclo" is used to indicate the presence of a ring in the structure of an alkane. The compounds in this image are cycloalkanes. For naming organic compounds, you must add cyclo to the beginning of alkanes with rings in the structure

Bridgeheads

The two carbon atoms where the rings are fused together in a bicyclic compound.

Gauche Conformations

The two conformations that exhibit gauche interaction in the previous image are called gauche conformations, and they are degenerate

3rd Step for Naming Bicyclic Compounds

Then number the parent, and assign a locant to each substituent. Start at one of the bridgeheads and continue numbering along the longest path that connects the bridgeheads. In the case in this image, we start at the lower bridgehead, so as to give the isopropyl group the lower number

Ring Flip

This process is not accomplished by simply flipping the molecule like a pancake. Rather, a ring flip is a conformational change that is accomplished only through a rotation of all C-C single bonds.

Conformation Analysis of Butane and Energy Diagram

Three highest energy conformations are the eclipsed ones, while the three lowest energy conformations are staggered conformations. Notice that one eclipsed conformation(where dihedral angle = 0°) is higher in energy than the other two exlipsed conformations. So they are not degenerate. Similarly, one staggered conformation(where dihedral angle = 180°) is lower in energy than the other two staggered conformations

Relative Stability of Isomeric Alkanes

To compare the stability of constitutional isomers, we look at the heat liberated when they each undergo combustion. By comparing the amount of heat given off by each combustion process, we can compare the potential energy that each isomer had before combustion. The isomer that produces less from combustion is lower in energy and therefore more stable

Calculating Energy Cost

We already assigned 4 kJ/mol to each pair of eclipsing H's. If we know that the torsional strain of propane is 14 kJ/mol, then it is reasonable to assign 6 kJ/mol to the eclipsing of an H and methyl group

Disubstituted Cyclohexane

When drawing chair conformations of a compound that has two or more substituents, we must consider the three-dimensional orientation, or configuration, or each substituent. In this image, the chlorine atom is on a wedge, which means that it is coming out of the page: it is UP. The methyl group in on a dash, which means that it is below the ring: or DOWN.

Comparing the Stability of Both Chair Conformations

When two chair conformations are in equilibrium, the lower energy conformation will be favored. In this image, consider the two chair conformations of methylcyclohexane. At room temperature, 95% of the molecules will be in the chair conformation that has the methyl group in an equatorial position because it is the lower energy conformation.

Drawing the Skeleton of a Chair Conformation

When you are finished drawing a chair, it should contain three sets of parallel lines. If your chair does not contain three sets of parallel lines, then it has been drawn incorrectly