Chapter 3 Structure and Stereochemistry of Alkanes

Nomenclature (3-3)ÚSince the names of all organic compounds

are based on alkanes, a brief summary of the rules of nomenclature is necessary.

Table 3-2 gives you the names of the first 20 common straight chain alkanes (also see slide 79).

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slide 79).

ÚSystematic Names or IUPAC names:

IUPAC: International Union of Pure and Applied Chemistry

ÚRule 1Find the longest continuous chain of C atoms and use the name of this chain as the base name of the compound.

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substituent(alkyl group)

Name the alkyl (+ position) + the main chain

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Name the alkyl (+ position) + the main chain

3-methylhexane or 4-methylhexane

Ú If there is a choice between 2 or more chains of equal length, use the one with the greater # of substituents, it simplifies the name.

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ÚRule 2number the longest chain beginning with the end of the chain nearest a substituent.You always want the positions of the substituents to be as low as possible.

1 7

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23

4 5

6 7 12

34

56

7

positions of substituents:3, 4, 5, 6 = 18

positions of substituents:2, 3, 4, 5 = 14

Rule 3Name the substituents attached to the longest chain as alkyl groups. Give the location of each alkyl group using the carbon # to which it is attached.

The name and the position must be separated by a hyphen.

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separated by a hyphen.

Use alphabetical order to name the substituents, not the number of their positions.

Rule 4 (multiple groups)When 2 or more of the same substituents are present, use prefixes di (2), tri (3), tetra (4), … to avoid naming them more than once.

(prefix such as di-, tri- tetra- etc are not used to determine the alphabetical order)

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12

34

56

7

3-ethyl-2,4,5-trimethylheptane

Ú Other common alkyl groups:3 carbons

4 carbons

CH3CH2CH2

propyl

CH3CH

CH3

isopropyl

CH3CH2CH2CH2 CH3CHCH2

CH3

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CH3CH2CH2CH2 CH3CHCH2

CH3CH2CH

CH3

CH3 C

CH3

CH3

butyl isobutyl

sec-butyl tert-butyl(t-butyl)

ÚClassification carbon atoms

C

H

H

R CR

R

H

C

R

R

R

primary (1o) secondary (2o) tertiary (3o)

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primary (1o)carbon

secondary (2o)carbon

tertiary (3o)carbon

ÚFunctional groupsWhen functional groups are present, the same rules apply but the longest chain must include the functional group (the carbon from chain) and the functional group must be numbered.

the carbon atom bearing the functionalgroup is included in the longest carbonchain

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O

OHchain

ÚReactions of alkanes (3-6)Alkanes are fairly unreactive. However 3 common reactions are listed below.

– Combustion

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CnH2n + 2 + O2 ∆

n CO2 + (n + 1) H2O

– Cracking makes smaller alkanes from large ones. Not specific, ie cannot predict which shorter alkanes will be produced.

C12H26

H2, ∆

catalyst C5H12 +

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C12H26

C7H16

– Halogenation

a more useful reaction. However it has the tendency to give more than one product. This reaction will be discussed further in chapter 4.

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Structure and conformation of Alkanes (3.7)– In alkanes, all “C” atoms are hybridized sp3.

– With more than one “C” atom, different arrangements are possible due to rotation.

– These arrangements formed by rotation along a

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– These arrangements formed by rotation along a single bond are called:

Conformations– And a specific conformation is caller a:

Conformer

ÚNewman Projections are normally used to represent conformations along C-C single bonds as seen below for ethane

Figure 3-5

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Ú There are many possible conformers of ethane. While they all exist, one is very stable (lower energy) while one is higher in energy (less favoured)

Figure 3-7

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Ú ө: dihedral angle (angle between C-H bond of the front carbon and C-H bond of the back carbon

Ú ө = Oo in eclipsed conformation (molecule in a higher energy state)

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Ú ө = 60o in staggered conformation (molecule in a lower energy state)

ÚBecause the energy is less for staggered conformation, they are favoured conformations.

Ú Increasing the carbon chain by one carbon (propane) make the staggered conformation even more favoured, since in the eclipsed conformer, a stronger interaction (steric factors) exist between the H and CH3.

Figure 3-9

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ÚWith more carbons on the chain, (ex: butane) even more possibilities exist.Let’s consider the C2-C3 bond:

Figure 3-10

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ÚThe energy associated with the rotation of one conformer to another one is called: torsional energy.

Figure 3-11

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§ The anti-conformation is the lowest energy conformation, followed by the gauche conformations and the eclipsed ones, and the highest energy conformation is always the fully eclipsed conformation.

§ Exercise:Looking at C2-C3 bond of hexane draw the Newman projections for the anti and gauche conformations

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ÚCycloalkanes (3-10)Whether a cycloalkane exists or not normally depends on the ring strain. Typically, the ring strain is too large on small rings for them to exist. We will focus on 5 and 6 carbon in the

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will focus on 5 and 6 carbon in the ring.

Cis-Trans Isomerism (3-11)Ú Similar to alkenes, cycloalkanes can have cis

and trans geometric isomersExample: 1,2-dimethyl cyclopentane

CH3H

CH3

H

H C

H

CH

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Ú cis: both substituents on the same side of the ringtrans: substituents on opposite sides of the ring

H CH3 H3C CH3trans cis

Conformations of Cyclohexanes (3-13)2 extreme conformations are possible because the ring is not flat (respect bond angle)

Chair Conformation:

Figure 3-19

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ÚBoat ConformationFigure 3-20

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ÚChair conformation is much more stable and usually preferred. This is due to less steric interactions in its conformation compared to the boat conformer.

Figure 3-21

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ÚTo draw chair conformation:– Draw two parallel lines slanted upward:

– Join the two parallel lines with a “V”

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– Join the two parallel lines with a “V”

– In any cyclohexane ring there are 6 axial positions. These are occupied by hydrogens or by other singly bonded atoms. The axial bonds are always vertical (up or down) following the general direction the “V” points towards. The other 6 positions are equatorial.

Figure 3-22

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ÚThere are 2 possible forms of the chair conformation. They are caused by “ring flip”. When flipping the chair, notice that the axial position and equatorial position change from one structure to the other.

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Ú In going from one chair to the other, the molecule will go through a series of 3 other conformations:

Figure 3-21

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Conformation of mono-substituted cyclohexanes (3-14)

Ú In mono-substituted cyclohexanes, the favored conformation is normally the one with the substituent at the equatorialposition. This is due to the steric factorsfound in the axial position.

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found in the axial position.Figure 3-23

ÚThese steric unfavourable interactions when a group is axial are called: 1,3-diaxial interactions.

Figure 3-26

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Ú In other words, it is better to have the substituent in an anti conformation rather than in the gauche conformation.

Figure 3-24

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Figure 3-25

Conformations of disubstituted cyclohexanes (3-15)

ÚWith 2 substituents present, the molecule can be either cis or trans:

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ÚProblem: Are the following cycloalkanes cis or trans?

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Ú With 2 substituents, it is recommended to write both possible conformers via “ring flip”.

Ú The conformer with the least amount of steric interactions is normally favored.

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Ú If 2 substituents of unequal sizes are present, the larger substituent usually prefer to occupy the equatorial position.

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§ Question:For the molecule of cis-1-Bromo-2-chlorocyclohexane, draw the two possible chair conformations and indicate the most stable.

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ÚQuestion:For the following molecule, draw the most stable chair conformation.

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Study ProblemsÚ 3-33

Which of the following structures represent the same compound?

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Ú 3-34Draw the following compounds.

3-ethyloctane

cis-1-ethyl-4-methylcyclohexane

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isobutylcyclopentane

3-44

Draw the two chair conformations of each of the compounds below. Circle the most stable.

cis-1-ethyl-3-methylcyclohexane

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trans-1-ethyl-4-methylcyclohexane

Ú 3-47Draw Newman projection along the C3-C4 bond to show the most stable conformation of 3-ethyl-2,4,4-trimethyl heptane.

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