Chapter 3 Structure and Stereochemistry of Alkanesfaculty.sdmiramar.edu/choeger/Chap 3 Alkanes.pdf · Chapter 3 Structure and Stereochemistry of Alkanes Alkenes: Structure and Stereochem

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Chapter 3Structure and Stereochemistry

of Alkanes

Alkenes: Structure and Stereochem Slide 3-2

Classification Review

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Alkane Structural Formulas

• All C-C single bonds• Saturated with hydrogens (no pi bonds)• Ratio: CnH2n+2; cycloalkanes will be different!• Straight chain. acyclic alkane homologs: CH3(CH2)nCH3

• Same ratio for branched alkanes (alkanes with carbonsattached to internal carbons of longest chain)

C

H

C

H

H

H C H

H

H

C H

H

H

Isobutane, C4H10

C

H

C

H

H

H C C

H

HH H

H

H

Butane, C4H10


Alkane Examples

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IUPAC Nomenclature• Find the longest continuous carbon chain.• Number the carbons, starting closest to the first branch.• Name the groups attached to the chain, using the carbon

number as the locator.• Alphabetize substituents.• Use di-, tri-, etc., for multiples of same substituent.

See nomenclature of organic molecules handout


Longest Chain• The number of carbons in the longest chain determines the

base name: methane, ethane, propane, butane, pentane,hexane, heptane, octane, nonane, decane, undecane. (for C1to C11)

• If there are two possible chains with the same number ofcarbons, use the chain with the most substituents.

C

CH3

CH2

CH3

CH CH2 CH2 CH3

CH CH2 CH3

H3C

H3C

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Number the Carbons• Start at the end closest to the first attached group.• If two substituents are equidistant, look for the next closest

group (rule of lowest sum of index numbers).

1

2

3 4 5

6 7CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3 2 + 3 + 6

vs

2 + 5 + 61

CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

23

45

67


Alkyl Groups• CH3-, methyl• CH3CH2-, ethyl• CH3CH2CH2-, n-propyl• CH3CH2CH2CH2-, n-butyl• etc

CH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3

tert-butyl

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Propyl Groups

C

H

H

H

C

H

H

C

H

H

H

n-propyl

C

H

H

H

C

H

C

H

H

H

isopropyl

H

A primary carbon A secondary carbon


Butyl Groups

C

H

H

H

C

H

C

H

H

C

H

H

H

C

H

H

H

C

H

C

H

HH

C

H

H

n-butyl sec-butyl

H

H

A primary carbon A secondary carbon

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Isobutyl Groups

CH

H

H

C

C

H H

C

H

H

H H

CH

H

H

C

C

H H

C H

H

H

H

H

H

A primary carbon A tertiary carbon

isobutyl tert-butyl


Alphabetize• Alphabetize substituents by name.• Ignore di-, tri-, etc. for alphabetizing.

CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

3-ethyl-2,6-dimethylheptane

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Complex Substituents• If the branch has a branch, number the carbons from the

point of attachment.• Name the branch off the branch using a locator number.• Parentheses are used around the complex branch name.

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3

1-methyl-3-(1,2-dimethylpropyl)cyclohexane


Physical Properties

• Solubility: hydrophobic• Density: less than 1 g/mL• Boiling points increase with increasing

carbons (little less for branched chains).• Melting points increase with increasing

carbons (less for odd-number of carbons).

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Boiling Points of AlkanesBranched alkanes have less surface area contact, so weakerintermolecular forces.


Melting Points of Alkanes

Branched alkanes pack more efficiently into a crystallinestructure, so have higher m.p.

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Branched Alkanes• Lower b.p. with increased branching• Higher m.p. with increased branching• Examples:

H

CH3CH

CH3

CH2 CH2 CH3

bp 60°Cmp -154°C

CH3CH

CH3

CHCH3

CH3 bp 58°Cmp -135°C

bp 50°Cmp -98°C

CH3 CC 3

CH3

CH2 CH3


Major Uses of Alkanes• C1-C2: gases (natural gas)• C3-C4: liquified petroleum (LPG)• C5-C8: gasoline• C9-C16: diesel, kerosene, jet fuel• C17-up: lubricating oils, heating oil• Origin: petroleum refining

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Reactions of Alkanes• Combustion

Complete oxidation of an organic molecule in the presence of oxygen:

long-chain alkanes catalyst

shorter-chain alkanes

CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +

heat or light

• Cracking and hydrocracking (industrial)

• Free-Radical HalogenationReaction of an organic molecule with X2; replaces H’s on sp3 carbons:

2 CH

3CH

2CH

2CH

3 + 13 O

2 !"! 8 CO

2 + 10 H

2O + heat


Conformers of Alkanes• Isomers that can be interconverted simply by rotations

around carbon-carbon bonds are called conformations.• All structures result from the free rotation of a C-C single

bond• Different conformations may differ in energy. The lowest-

energy conformer is most prevalent (highest population).• Typical molecules constantly rotate through all the possible

conformations, even though the populations of each maydiffer widely.

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Conformational Analysis Vernacular• How to represent (draw) conformations?

Sawhorse and Newman ProjectionsConsider an ethane-like species (G1CH2-CH2G2)

Hb

G1

Ha

Hd

G2

Hc

G1

HaHb Hc

Hd

G2

G1

Hb Ha

G2

Hd Hc

Eclipsed Conformation

G1

Hb Ha

G2

HdHc

Staggered Conformation

clinalclinal

periplanar

periplanar

syn

anti

G G

G3b

G1b

G2b

G2f

G1f

G3f

gauche

(60o)

antigauche


Ethane Conformers• Staggered conformer has lowest energy.• Dihedral angle = 60 degrees

H

H

HH

H H

Newmanprojection

sawhorse

model

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Ethane Conformers (2)• Eclipsed conformer has highest energy• Dihedral angle = 0 degrees (drawn as below for convenience)


Conformational Analysis• Torsion: energy required to move one group past another

in space. Based primarily on sterics.• Torsional strain: resistance to rotation.• For ethane, only 12.6 kJ/mol

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Propane Conformers

Note slight increase in torsional strain; due to the more bulky(“sterically demanding”) methyl group.


Butane Conformers C2-C3• Highest energy has methyl groups eclipsed.• Steric hindrance• Dihedral angle = 0 degrees

totally eclipsed

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Butane Conformers (2)• Lowest energy has methyl groups anti.• Dihedral angle = 180 degrees

anti


Butane Conformers (3)• Methyl groups eclipsed with hydrogens• Higher energy than staggered conformer• Dihedral angle = 120 degrees

eclipsed

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Butane Conformers (4)• Gauche, staggered conformer• Methyl groups closer than in anti conformer• Dihedral angle = 60 degrees

gauche


Conformational Analysis

NOTE: more common to go anti to total eclipse to anti (lowest to highest to lowest

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Higher Alkanes

• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag (line-angle represents

this).

CH3CH2CH2CH2CH3

C

HCCCC

H H H H

H H

HH

HH

H


Cycloalkanes

• Rings of carbon atoms (-CH2- groups)• Formula: CnH2n (same as alkenes)• Nonpolar, insoluble in water• Compact shape• Melting and boiling points similar to branched alkanes with

same number of carbons• Conformational analysis important

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Naming Cycloalkanes

• Cycloalkane usually base compound• Number carbons in ring if >1 substituent.• First in alphabet gets lowest number.• May be cycloalkyl attachment to chain.

CH2CH3

CH2CH3

CH3


Cis-Trans Isomerism

• Due to restricted rotation caused by ring• Cis: like groups on same side of ring• Trans: like groups on opposite sides of ring

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Cycloalkane Stability

• 5- and 6-membered rings most stable• Bond angle closest to 109.5°• Must consider concept of ring strain• Torsional strain due to eclipsing interactions• Angle (Baeyer) strain• Measured by heats of combustion per -CH2 -


Heats of Combustion per CH2(Alkane + O2 → CO2 + H2O) ÷ (#CH2’s)

Long-chainalkane

658.6 kJ 658.6697.1 686.1

664.0 663.6 kJ/mol662.4

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Cyclopropane• Large ring strain due to angle compression• Very reactive, weak bonds


Cyclopropane (2)Torsional strain because of eclipsed hydrogens

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Cyclobutane• Angle strain due to compression• Torsional strain partially relieved by ring-puckering

Slight increase in angle strain relieves some torsional strain!


Cyclopentane

• If planar, angles would be 108°, but all hydrogens would beeclipsed.

• Puckered conformer reduces torsional strain.

“Envelope”conformation;

pseudo-rotation

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Cyclohexane• Combustion data shows it is unstrained.• Angles would be 120°, if planar.

• The chair conformer has 109.5° bond angles and all hydrogensare staggered.

• No angle strain and no torsional strain. HOW?


Chair Conformer

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Drawing Chair Conformations

A B

C D

Start by drawing two parallel, butoffset lines (A). Put a nose on it(B), followed by a tail (C). Finally,add in the axial and equitorial bonds(D). Process starts with offsetparallel lines running in otherdirection for ‘chair flip’conformation. When transcribinggroups from a cyclohexyl structureto the chair conformation, rememberthe relationship of axial andequitorial groups (and don’t mix thisup with cis and trans!!). Practicewill make perfect!


Boat Conformer

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Conformational Energy


Axial and Equatorial Positions

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Monosubstituted Cyclohexanes


1,3-Diaxial Interactions

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Disubstituted Cyclohexanes


Cis-Trans IsomersBonds that are cis, alternate axial-equatorial around the ring.

CH3

CH3

One axial, one equatorial

a

e

a

e

a

e

a

e

a

e

a

e

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Bulky Groups• Groups like t-butyl cause a large energy difference between the

axial and equatorial conformer.• Most stable conformer puts t-butyl equatorial regardless of other

substituents.


Conformational Energies: A Values• Used to quantitate 1,3-diaxial interactions;• Defined as the the energy released when a group goes

FROM an axial position TO an equitorial position;• Thermodynamically: A = -ΔGo; as a result, A values are

positive for a thermodynamically-favorable conformationalchange;

• Additive; Atot can be used to estimate equilibrium quantities

A value (kJ) = –!G

X

°= RT ln K

eq= 8.314 " 10

–3kJ/K( )T ln

II

IX

HX

H

Keq

I II

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Typical AValues

X A Value

F 0.63

Cl 1.80

Br 1.59

I 1.80

OH 3.64

OCH3 2.51

OCD3 2.34

OC2H5 3.77

OAc 2.51

OC(O)CF3 2.85

OCHO 1.13

OTs 2.09

-ONO2 2.47

SH 3.77

SCN 5.15

SCH3 2.93

SC6H5 3.35

-S- 5.44

SOC6H5 7.95

SOCH3 5.02

SO2C6H5 10.46

SO2CH3 10.46

X A Value

-CN 0.71

-NC 0.88

-NCO 2.13

-NCS 1.17

-N=C=N-R 4.18

NH2 6.69

NHCH3 4.18

N(CH3)2 8.79

-NH3+ 7.95

-NO2 4.60

PH2 6.69

P(CH3)2 6.28

PCl2 7.95

P(OCH3)2 6.28

P+(CH3)3 >12.55

P(S)(CH3)2 >12.55

HgBr -1.26

HgCl 1.26

X A Value

CH3 7.11

CF3 8.79

CH2CH3 7.32

CH=CH2 5.65

C!H 1.72

CH2C(CH3)3 8.37

CH2OTs 7.32

CH(CH3)2 9.00

c-C6H11 9.00

C(CH3)3 >16.74

C6H5 12.55

CO2H 5.65

CO2- 8.03

CO2CH3 5.31

CO2C2H5 5.02

CO2CH(CH3)2

4.02

COCl 5.23

COCH3 4.90

Si(CH3)3 ------

Ge(CH3)3 8.79

Sn(CH3)3 4.60

Pb(CH3)3 2.93


Determination of Atotal and Keq

So at equilibrium: 71% I and 29% II are presentNote: -ACl and -ABr were used because they went from equitorial to axial!

CH3

H

CH3

H

Keq

I II

Cl

H

Br

H

Cl

H

Br

H

ATot

= ACH

3

+ – ACl

( )+ – ABr

( ) = 7.11 – 1.80 – 1.59 = 3.72 kJ

At 25°C: ln Keq= ln

II

I=

3.72

8.314 ! 10–3( ) 298( )

= 1.50

Keq= 0.41 =

II

I

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Bicyclic Alkanes• Fused rings share two adjacent carbons.• Bridged rings share two nonadjacent C’s.

bicyclo[3.1.0]hexanebicyclo[3.1.0]hexane bicyclo[2.2.1]heptanebicyclo[2.2.1]heptane


Cis- and Trans-Decalin• Fused cyclohexane chair conformers• Bridgehead H’s cis, structure more flexible• Bridgehead H’s trans, no ring flip possible.

H

H

cis-decalin

H

H

trans-decalin

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Bicyclo[4.4.0]decane


Chapter 3 Homework34, 35, 37, 39, 42-44, 46 plus:

47) Draw the most stable chair conformation for the each ofthe following, and calculate the percentages of each presentat equilibrium:

Cl

I

Br

F

OCH3a) b)

Documents

Chapter 3 Structure and Stereochemistry of Alkanesfaculty.sdmiramar.edu/choeger/Chap 3 Alkanes.pdf · Chapter 3 Structure and Stereochemistry of Alkanes Alkenes: Structure and Stereochem