3.7 Solvating Effects. The efficiency by which H2O can solvate H+ and A– can affect the pKa of H–A 3.8 Counterions (please read) 3.9 Lewis Acids and Bases

Acid - an electron pair acceptor Base - an electron pair donor.  

51  

Chapter 4: Alkanes and Cycloalkanes 4.1 Introduction to alkanes Hydrocarbon: molecules that contain only carbon and hydrogen

1. Aliphatic – open chain hydrocarbons a. alkanes - contain C-C single bonds - CnH(2n+2) b. alkenes - contain C=C double bonds - CnH(2n) c. alkynes - contain CΞC triple bonds - CnH(2n-2)

2. Cycloalkanes - CnH(2n)

3. Arenes (aromatics) - cyclic hydrocarbons with alternating C-C single and double bonds

C C CC

H

H H

H

C

H

H

H

H

H

H

H

H

hexane 1-hexene1-hexyne benzenecyclohexane

saturated hydrocarbons

unsaturated hydrocarbons

52  

26

53

4.2 Nomenclature of Alkanes Systematic Nomenclature (IUPAC System)

Prefix-Parent-Suffix

Parent- number of carbons Prefix- substituents

Suffix- functional groups

Naming Alkanes General Formula: CnH(2n+2) suffix: -ane Parent Names: (Table 4.1, p. 141)

1 CH4 Methane CH4 2 CH3CH3 Ethane C2H6 3 CH3CH2CH3 Propane C3H8 4 CH3(CH2)2CH3 Butane C4H10 5 CH3(CH2)3CH3 Pentane C5H12 6 CH3(CH2)4CH3 Hexane C6H14 7 CH3(CH2)5CH3 Heptane C7H16 8 CH3(CH2)6CH3 Octane C8H18 9 CH3(CH2)7CH3 Nonane C9H20 10 CH3(CH2)8CH3 Decane C10H22

54

Alkyl substituents (group): carbon chains which are a substructure of a molecule

one carbon group off a main chain

R= Rest of the molecule (mainchain) 1 CH3-R Methyl 2 CH3CH2-R Ethyl 3 CH3CH2CH2-R Propyl 4 CH3(CH2)2CH2-R Butyl 5 CH3(CH2)3CH2-R Pentyl 6 CH3(CH2)4CH2-R Hexyl 7 CH3(CH2)5CH2-R Heptyl 8 CH3(CH2)6CH2-R Octyl 9 CH3(CH2)7CH2-R Nonyl 10 CH3(CH2)8CH2-R Decyl

Table 4.2 (p. 143)

27

55

Rules for Systematic Nomenclature of Alkanes 1.  Find the parent chain a. Identify the longest continuous carbon chain as the

parent chain. b. If more than one different chains are of equal length

(number of carbons), choose the one with the greater number of branch points (substituents) as the parent.

CH3 CHHC

CH2

CH2

CH2 CH2

CH3

CH3

CH3

7 carbons = hept-

CH3 CHHC

CH3

CH2

CH2 CH3

CH2 CH3 CH3 CH CH

CH3

CH2

CH2 CH3

CH2 CH3

2 branch pts. 1 branch pt.

56

2.  Numbering the carbons of the parent chain a. Number the carbon atoms of the parent chain so that any branch points have the lowest possible number

b. If there is branching equidistant from both ends of the parent chain, number so the second branch point has the lowest number.

CH3 CHHC

CH2

CH2

CH2 CH2

CH3

CH3

CH3

CH3 CHHC

CH2

CH2

CH2 CH2

CH3

CH3

CH3

1

2

3

7 123

45

5

4

6

6

7

branch pts. at carbons 3 and 4 branch pts. at carbons 4 and 5

4CH3 CH CH2

CH2

CH2 CH CH

CH3

CH2 CH3

1 2

3 5 6 7 8 9

H3C CH2 CH3

6CH3 CH CH2

CH2

CH2 CH CH

CH3

CH2 CH3

9 8

7 5 4 3 2 1

H3C CH2 CH3

branch pts. at carbons 3, 6, 7 branch pts. at carbons 3,4,7

28

57

3. Substituents a. Identify and number the substituents and list

them in alphabetical order.

b. If there are two substituents on the same carbon, assign them the same number.

4. Write out the name a. Write out the name as a single word:

hyphens (-) separate prefixes commas (,) separate numbers

b. Substituents are listed in alphabetical order c. If two or more identical substituents are present use the

prefixes: di- for two tri- for three tetra- for four

6CH3 CH CH2

CH2

CH2 CH CH

CH3

CH2 CH3

9 8

7 5 4 3 2 1

H3C CH2 CH3Parent C-9 = nonane3- ethyl4-methyl7-methyl 4,7-dimethyl

58

note: these prefixes (di-, tri-, tetra-, etc.) are not used for alphabetizing purposes

6CH3 CH CH2

CH2

CH2 CH CH

CH3

CH2 CH3

9 8

7 5 4 3 2 1

H3C CH2 CH3

3- ethyl-4,7-dimethylnonane

5.  Complex Substituents (substituents with branching) a. Named by applying the four previous rules with some modification b. Number the complex substituent separately from the parent. Begin numbering at the point of attachment to the parent

chain c. Complex substituents are set off by parenthesis.

CH3 CH CH2

CH3

CH2 CH CH CH2 CH2 CH2

H2C CH2

CH3

CH3

CH3

1 2 3 4 5 6 7 8 9 10

1 2 3 2,6-dimethyl-5-(1-methylpropyl)decane

CH2

29

59

Nonsystematic (trivial) Names: 3-carbons: 4-Carbons: 5- Carbons: Alphabetizing trivial names: Iso- and neo are part of the alkyl group name and are used for alphabetizing. sec- and tert- are not included in the alphabetical order.

4-(1-methylethyl)heptane-or-

4-Isopropylheptane

2-methyl-6-(2-methylpropyl)decane-or-

6-Isobutyl-2-methyldecane

CH

H3C

H3C

Isopropyl-(1-methylethyl)

Parent Chain

CH

CH2

H3C

Parent Chain

sec-butyl-(1-methylpropyl)

Isobutyl-(2-methylpropyl)

C

CH3

CH3

tert-butyl-(1,1-dimethylethyl)

H3C

CH2Parent Chain

CH3

CHH3C H3CParent Chain

C

CH3

CH3

tert-pentyl-, tert-amyl(1,1-dimethylpropyl)

CH2Parent ChainH3CCH2C Parent

ChainH3C

CH3

CH3neopentyl-

(2,2-dimethylpropyl)Isopentyl-, isoamyl

(3-methylbutyl)

CH2 Parent Chain

CH2CH

H3C

H3C

60

ParentChain

Propane Cyclopropane cyclopropyl

ParentChain

ParentChain

Butane Cyclobutane Cyclobutyl

ParentChain

Pentane Cyclopentane

Hexane Cyclohexyl

Cyclopentyl

Cyclohexane

Cycloalkanes

Heptane

ParentChain

Cycloheptane Cycloheptyl

ParentChain

ParentChain

ParentChain

Octane Cyclooctane Cyclooctyl

Nonane Cyclononane Cyclononyl

Decane Cyclodecane Cyclodecyl

30

61

Naming Cycloalkanes General Formula: CnH(2n) 1.  Parent Chain a. Use the cycloalkane as the parent chain if it has a greater number of carbons than any alkyl substituent.

b. If an alkyl chain off the cycloalkane has a greater number of carbons, then use the alkyl chain as the parent and the cycloalkane as a cycloalkyl- substituent.

CH3

Methylcyclopentane 2-Cyclopropylbutane

CH3

CH3

12

34

5

6

CH3

CH3

12

3

45

6

1,3-Dimethylcyclohexane-not-

1,5-Dimethylcyclohexane

CH31

2

345

6

CH3

12

3

45

6

CH3 CH3

CH3CH3

1,2,4-Trimethylcyclohexane(1 + 2 + 4 = 7)

-not-1,3,4-Trimethylcyclohexane

(1 +3 + 4 = 8)

2. Numbering the Cycloalkane a. When numbering the carbons of a cycloalkane, start with a substituted carbon so that the substituted carbons have the lowest numbers (sum).

62

2. b. When two or more different substituents are present, number according to alphabetical order.

1

2 1

2

1-Ethyl-2-methylcyclohexane-not-

2-Ethyl-1-methylcyclohexane

CH3

Cl

1-Chloro-2-methylcyclobutane

3. Halogen Substituents Halogen substituents are treated exactly like alkyl groups: -F fluoro- -Cl chloro- -Br bromo-

-I iodo-

31

63

Degrees of Substitution Primary (1°) Carbon: carbon that is bonded to only one other carbon Secondary (2°) Carbon: carbon that is bonded to two other carbons Tertiary (3°) Carbon: carbon that is bonded to three other carbons Quarternary (4°) Carbon: carbon that is bonded to four other carbons

1° Hydrogens- hydrogens on a primary carbon. -CH3 (methyl group) 2° Hydrogens- hydrogens on a secondary carbon. -CH2- (methylene group) 3° Hydrogens- hydrogens on a tertiary carbon. CH (methine group)

HO

Primary (1°ˇ) Secondary (2°ˇ) Tertiary (3°ˇ)Quarternary (4°ˇ)

secondaryalcohol

H3C CH2 CH

CH3

CH2 C

CH3

CH3

CH3

methyl group: 1° hydrogens methylene group: 2° hydrogens methine group: 3° hydrogens

4.3 Constitutional Isomers of Alkanes Isomers: compounds with the same chemical formula, but different arrangement of atoms

Constitutional isomer: have different connectivities (not limited to alkanes)

straight-chain or normal hydrocarbons branched hydrocarbons

n-butane n-pentane

C4H10 C5H12

n-butane isobutane n-pentane isopentane neopentane

C2H6O

OHbutanol

O

diethyl ether

64  

32

4.4 Relative Stability of Isomeric Alkanes Combustion of hydrocarbons (Oxidation) CnH2n+2 + O2 n CO2 + (n+1) H2O + heat (Δ)

Heat (ΔH°) of combustion = ΔH°(products) − ΔH°(reactants) measure of relative stability

65  4.5 Sources and Uses of Alkanes (please read)

+ 12 1/2 O2

+ 12 1/2 O2

+ 12 1/2 O2

8 CO2 + 9 H2O + 5470 KJ/mol

8 CO2 + 9 H2O + 5460 KJ/mol

8 CO2 + 9 H2O + 5452 KJ/mol

C8H18

C8H18

C8H18

4.6 Drawing Newman Projections Stereochemistry: three-dimensional aspects of molecules

Conformation: different spatial arrangements of atoms that result from rotations about single (σ) bonds

Conformer: a specific conformation of a molecule

Sawhorse

HH

HH

H H

H H

H

H

H

H

C C

H

HHH

HH H

H HH

HHFrontcarbon

Backcarbon

Newman projection

66  

33

Staggered Eclipsed

4.7 Conformational Analysis of Ethane and Propane There are two conformations of ethane:

Newman Projections Dihedral (torsion) angle: angle between an atom (group) on the front atom of a Newman Projection and an atom (group) on the back atom

Dihedral angles of ethane: Staggered conformation: 60° (gauche), 180° (anti), and 300° (-60°, gauche) Eclipsed conformation: 0°, 120°, and 240° (-120°) 67  

68  

Energy vs. dihedral angle for ethane http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch03/ch3-03.html#methane

The barrier (Eact) for a 120° rotation of ethane (from one staggered conformer to another) is 12 KJ/mol. The eclipsed conformer is the barrier to the rotation. An H–H eclipsing interaction = 4 KJ/mol

Torsional Strain: strain (increase in energy) due to eclipsing interactions

Eclipsed Staggered

34

69  

The barrier to C-C rotation for propane is 14 KJ/mol = 1 (H – CH3) + 2 (H – H) eclipsing Interactions. A H – CH3 eclipsing interaction is 6 KJ/mol

eclipsed

staggered

Conformations of Propane

6.0 KJ/mol

H H

H3C

H

H

H

C C

H

H3CHH

HH H

H HH3C

HH

H

H

H

H3C

HH4.0 KJ/mol 4.0 KJ/mol

Energy vs. dihedral angle for propane

Eclipsed Staggered

70  

35

4.8. Conformational Analysis of Butane http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch03/ch3-04.html#butane Two different staggered and eclipsed conformations Staggered: anti

Staggered: gauche

H H

H3C

H

H

CH3

C C

H

H3CH3C

H

HH

H H

H3C

H

CH3

H

C C

H3C

H3CHH

HH

gauche CH3 – CH3

dihedral angle = 60°

anti CH3 – CH3

dihedral angle = 180°

3.8 KJ/mol

H

H3C HH3C

HH

CH3

H HH3C

HH

71  

Steric Strain: repulsive interaction that occurs when two groups are closer than their atomic radii allow

H

H3C HH3C

HH

Eclipsed conformations of butane: rotational barrier of butane is 19 KJ/mol. A CH3-CH3 eclipsing interaction is 11 KJ/mol.

CH3 - H CH3 - CH3

H

H

CH3

H3C

HH H

CH3

H

H3C

HH4.0 KJ/mol 4.0 KJ/mol

4.0 KJ/mol

11.0 KJ/mol

6.0 KJ/mol

6.0 KJ/mol

16.0 KJ/mol 19.0 KJ/mol 72  

36

73  

Energy vs. dihedral angle for butane

Summary (Table 4.6, p. 164) H - H eclipsed 4.0 KJ/mol torsional strain H - CH3 eclipsed 6.0 KJ/mol mostly torsional strain CH3 - CH3 eclipsed 11 KJ/mol torsional + steric strain CH3 - CH3 gauche 3.8 KJ/mol steric strain

74  

4.9 Cycloalkanes - Are cycloalkanes planar or nonplanar? Angle Strain: strain due to deforming a bond angle from its ideal value (Baeyer Strain Theory) Prediction is that cyclopentane is strain-free, while all other cycloalkanes will be strained

60° 90° 108° 120° 128° 135°

Internal angles of polygons

CnH2n + 3n/2 O2 n CO2 + n H2O + heat cycloalkane (can be measured)

Total Strain Energy =

Sample ΔHcomb per -CH2-

_ Reference ΔHcomb per -CH2- • n

Heats of Combustion of Cycloalkane: the more strained a compound is, the more heat it releases upon combustion

37

75

Total Strain Energy =

Sample ΔHcomb per -CH2-

_ Reference ΔHcomb per -CH2- • n

strained  rings  

common  rings  

medium  rings  

large  rings    (>12)  

Combustion analysis of cycloalkanes do not agree with Baeyer’s prediction.

With the exception of cyclopropane, cycloalkane are not planar.

76  

Cyclopropane: reduced overlap of the sp3-hybridized orbitals

60° 109°

H

H

CH2

H

H

Total strain for cyclopropane

= angle strain + torsional strain

all adjacent CH2 groups are eclipsed

38

77  

Cyclobutane: reduced angle and torsional strain relative to cyclopropane

CH2

H

H

H

H

CH2

Puckering partially relieves torsional strain

Cyclopentane: planar conformation is strain free according to Baeyer. Although much less strained than cyclopropane or cyclobutane, there is considerable torsional strain (10 H–H eclipsing interactions). Envelope and half-chair conformations relieve much of the torsional strain

78  

4.10 Conformations of Cyclohexane. ΔHcomb suggests that cyclohexane is strain-free; favored conformation is a chair.

All H–H interactions are staggered - no torsional strain; minimal angle strain (~111°)

Other conformations of cyclohexane: half chair; twist boat, and boat

Boat: 4 H–H eclipsing interactions (torsional strain)

“Flagpole” interaction (steric strain)

39

79  

Chair cyclohexane has two types of hydrogens: axial: C-H axis is “perpendicular” to the “plane of the ring” equatorial: C-H axis is “parallel” to the “plane of the ring”

Chair cyclohexane has two faces; each face has alternating axial and equatorial -H’s

axial  equatorial

ae

a

e

a

a

e

e

a

e

a

e

top bottom

80  

chair chair

boat

twist-boat

twist-boat

half-chair half-chair

Conformation of cyclohexane vs energy http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch03/ch3-06.html#ring-flipping

40

81  

Boat (+ 30 KJ/mol)

Half-chair (+ 45 KJ/mol)

Chair Half-chair

(+ 45 KJ/mol)

Twist-boat (+23 KJ/mol)

Twist-boat (+ 23 KJ/mol)

Chair

Chair-Chair Interconversion of Cyclohexane

axial equatorial

axial equatorial

82  

4.11 Drawing Chair Cyclohexane – It is very important to draw chair cyclohexane properlly; so, practice, practice practice! Chair cyclohexane has three sets of parallel lines

h@p://www.masterorganicchemistry.com/2014/05/20/how-­‐to-­‐draw-­‐a-­‐cyclohexane-­‐chair/  

41

83  

4.12 Monosubstituted Cyclohexane – most stable chair conformation has the substituent in the equatorial position.

R

H

H

RKeq

axial equatorial

1,3-diaxial interactions

R= -CH3 5 : 95

84  

Methylcyclohexane equatorial

Methylcyclohexane axial

anti butane

2 gauche butane interactions: 2 x 3.8 KJ/mol = 7.6 KJ/mol

Axial position is more sterically congested (steric strain) and is therefore less favored thermodynamically

gauche butane +3.8 KJ/mol

42

85

R

H

H

R

Keq

ΔE = -RT ln Keq, where R= 8.3 x 10-3 KJ/mol, T= 300 °K (room temp)

Table 4.8, p 175 (expanded)

86  

4.13 Disubstituted Cyclohexane Relative orientation (above or below the cyclohexane ring) of the substituents

CH3

H

CH3

HCH3

H

CH3

H

H

CH3H

H3C CH3

HCH3

H

H

CH3

CH3

H

CH3

H

H

H3C

CH3

CH3

CH3

CH3

CH3

CH3

43

87  

4.14 cis-trans Stereoisomerism Stereochemistry: three-dimensional arrangement of atoms

(groups) in space Isomers: different chemical compounds with the same formula

Constitutional isomers: same formula, but different connectivity of atoms (or groups)

Stereoisomers: same connectivity, but different spatial arrangement of atoms or groups

ethylcyclopropane 1,2-dimethylcyclopropane

C5H10

CH3HH

H3C HCH3H

H3C

trans: on opposite side (face) of the ring cis: on the same side (face) of the ring

cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane

ΔHcomb is ~ 5 KJ/mol higher for the cis isomer

88  

cis-1,2-dimethylcyclohexane vs trans-1,2-dimethylcyclohexane

cis (one equatorial, one axial)!(2  x  3.8)  +  3.8  =  11.4  KJ/mol  

CH3HCH3

H

HH

HCH3

HCH3

HCH2

CH3H2C CH3

H

trans (two equatorial, no axial)!3.8  KJ/mol  

trans (no equatorial, two axial)!2  (2  x  3.8)  =  15.2  KJ/mol  

ΔG = 11.4 KJ/mol Keq = ~ 99:1

HCH3CH3

H

CH3HH

CH3

HH

HH

CH3CH2

HH2C CH3

H

44

89  

Haworth Projection – the cycloalkane is draw as a being planar and is oriented perpendicular to the page. Bond to substituents off the ring atoms are draw vertically - above the ring is up, below the ring is down.

CH3CH3

CH3

CH3

H

H

H

H

CH3CH3

CH3

H

CH3

H

H

H

90  

3.15: Polycyclic Systems - contains more than one ring fused - two rings share a pair of bonded atoms spirocyclic - one atom common to two rings bridged bicyclic - nonadjacent atoms common to two rings

fused spiro bridged

cis- and trans-decalin are stereoisomers, they do not interconvert into each other

H

H

H

HH

H

H

Htrans-decalin cis-decalin

45

91  

Drawing Structures CYCLIC ALKANES: Substituents on a cycloalkane can be cis or trans to each other. You should draw the ring in the plane of the paper (solid lines) and use dashes and wedges to show whether substitutents are above or below the plane of the ring.

correct incorrect

On occasion you may wish to distinguish the faces of a cycloalkane.

• •• •

cis trans

top face

bottom face

a

b

a

bb b

ba a

a

92  

CYCLOHEXANE: For cyclohexanes you may be asked to draw a chair, in which case all substituents must be either axial or equatorial. The following is the correct way to draw chair cyclohexane. Note how the axial and equatorial substituents are represented off each carbon.

Disubstituted chair cyclohexanes: correct   incorrect • •

• •

a

a

ee

a

a

e

e

a

e

a

e

trans trans cis

trans cis cis

trans trans cis

46