2 | 1

Chapter 2:

Alkanes and Cycloalkanes;

Conformational and Geometric

Isomerism

Alkanes

• Alkanes are saturated hydrocarbons, containing only carbon–carbon single bonds.

• Cycloalkanes contain rings. • Unsaturated hydrocarbons contain

carbon–carbon double or triple bonds. Aromatic hydrocarbons are cyclic compounds structurally related to benzene.

2 | 2

2 | 3

Alkane bonds

2 | 4

Three-dimensional models of ethane, propane, and butane.

2 | 5

Names and Formulas of the First Ten Unbranched Alkanes

2 | 6

11 undecane12 dodecane13 tridecane14 tetradecane15 pentadecane16 hexadecane17 heptadecane18 octadecane19 nonadecane20 icosane

2 | 7

• All alkanes fit the general molecular formula CnH2n+2

• Unbranched alkanes are called normal alkanes, or n-alkanes.

• -CH2- group is called a methylene group.

Alkanes

IUPAC Rules for naming Alkane

2 | 8

The root name of an alkane is that of the longest continuous chain of carbon atoms.

Substituents are groups attached to the main chain of a molecule.

Saturated substituents containing only C and H are called alkyl groups.

The one-carbon alkyl group derived from methane is called a methyl group.

2 | 9

2 | 10

2 | 11

Alkyl and Halogen Substituents

2 | 12

The two-carbon alkyl group is the ethyl group. The propyl group and the isopropyl group are three-carbon groups attached to the main chain by the first and second carbons, respectively.

2 | 13

R is the general symbol for an alkyl group.

• The formula R-H herefore represents any alkane,

• The formula R-Cl stands for any alkyl chloride (methyl chloride, ethyl chloride, and so on).

• Halogen substituents are named by changing the -ine ending of the element to -o.

2 | 14

2,2,4-trimethylpentane

2 | 15

Examples of Use of the IUPAC Rules

1 3 5 72

(6-Ethyl-2-methyloctane)

4 6 8

NOT

8 6 4 27

(3-Ethyl-7-methyloctane)

5 3 1

NOT

1 3 5 72

(2-Methyl-6-ethyloctane)

4 6 8

Rule (Cont’d)

2 | 16

5. When two substituents are present on the same carbon, use that number twice

1 3 5 72

(4-Ethyl-4-methyloctane)

4 6 8

Rule (Cont’d)

2 | 17

6. For identical substituents, use prefixes di-, tri-, tetra- and so on

6 4 25

(2,4-Dimethylhexane)

3 1

Rule (Cont’d)

1 3 52

(3,5-Dimethylhexane)

4 6

NOT

7 5 36

(2,4,5-Trimethylheptane)

4 21

NOT

1 3 52

(3,4,6-Trimethylheptane)

4 67

2 | 18

7. When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents

6 4 25

(2,3,5-Trimethyl-4-propylheptane)

317

Rule (Cont’d)

NOT 6

4 2

5

(only three substituents)

31

7

2 | 19

8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference

5 3 14

(2,3,5-Trimethylhexane)

26

Rule (Cont’d)

NOT

2 4 63

(2,4,5-Trimethylhexane)

51

2 | 20

Example 1

4 2

6

3 1

5 7or

4 6

2

5 7

3 1

● Find the longest chain as parent

2 | 21

Example 1 (Cont’d)

4 2

6

3 1

5 7instead of

4 6

2

5 7

3 1

● Substituents: two methyl groups dimethyl

● Use the lowest numbering for substituents

4 6

2

5 7

3 12 | 22

Example 1 (Cont’d)

● Complete name

4 6

2

5 7

3 1

(3,4-Dimethylheptane)

2 | 23

Example 2

2 | 24

Example 2 (Cont’d)

6-carbon chain

● Find the longest chain as parent

8-carbon chain 8-carbon chain

2 | 25

Example 2 (Cont’d)

● Find the longest chain as parent

9-carbon chain(correct!)

⇒ Nonane as parent

2 | 26

Example 2 (Cont’d)

● Use the lowest numbering for substituents

1

23 4

5 6

78

9

9

87 6

5 4

32

1

instead of

(3,4,7) (3,6,7)

2 | 27

Example 2 (Cont’d)

● Substituents 3,7-dimethyl 4-ethyl

1

23 4

5 6

78

9

2 | 28

Example 2 (Cont’d)

● Substituents in alphabetical order Ethyl before dimethyl

(recall Rule 4 – disregard “di”)● Complete name

1

23 4

5 6

78

9

(4-Ethyl-3,7-dimethylnonane)2 | 29

3C. How to Name Branched AlkylGroups

For alkanes with more than two carbon atoms, more than one derived alkyl group is possible

Three-carbon groups

Propyl Isopropyl(or 1-methylethyl)

2 | 30

Four-carbon groups

tert-butyl(or 1,1-dimethylethyl)

sec-butyl(1-methylpropyl)

Butyl Isobutyl

2 | 31

A neopentyl group

neopentyl(2,2,-dimethylpropyl)

2 | 32

Example 1

2 | 33

Example 1 (Cont’d)

(a)

(c)

(b)

(d)

● Find the longest chain as parent

6-carbonchain

7-carbonchain

8-carbonchain

9-carbonchain

2 | 34

(d)

⇒ Nonane as parent

1 3 5 7 92 4 6 8 9 7 5 3 18 6 4 2or

Example 1 (Cont’d)

● Find the longest chain as parent

2 | 35

Example 1 (Cont’d)

● Use the lowest numbering for substituents

5,6 4,5(lower numbering)

⇒ Use 4,5

1 3 5 7 92 4 6 8 9 7 5 3 18 6 4 2or

2 | 36

Example 1 (Cont’d)

● Substituents Isopropyl tert-butyl

9 7 5 3 18 6 4 2

⇒ 4-isopropyl and 5-tert-butyl

2 | 37

Example 1 (Cont’d)

● Alphabetical order of substituents tert-butyl before isopropyl

● Complete name

9 7 5 3 18 6 4 2

5-tert-Butyl-4-isopropylnonane2 | 38

Example 2

2 | 39

Example 2 (Cont’d)

(a)

(c)

(b)

● Find the longest chain as parent

8-carbonchain

9-carbonchain

10-carbonchain

⇒ Decane as parent

2 | 40

Example 2 (Cont’d)

1 3 5 7 92 4 6 8 10

10 8 6 4 29 7 5 3 1

or

2 | 41

1 3 5 7 92 4 6 8 10

10 8 6 4 29 7 5 3 1

or

Example 2 (Cont’d)

● Use the lowest numbering for substituents

5,6

⇒ Determined using the next Rules

5,6

2 | 42

Example 2 (Cont’d)

● Substituents sec-butyl Neopentyl

But is it● 5-sec-butyl and 6-neopentyl or● 5-neopentyl and 6-sec-butyl ?

2 | 43

Example 2 (Cont’d)

● Since sec-butyl takes precedence over neopentyl 5-sec-butyl and 6-neopentyl

● Complete name

10 8 6 4 29 7 5 3 1

5-sec-Butyl-6-neopentyldecane2 | 44

Physical Properties of Alkanes and Nonbonding Intermolecular Interactions

2 | 45

Hydrogen Bonding: (a) polar water

molecule: ball-and-stick model

Hydrogen Bonding: (b) hydrogen bonding

between water molecules

2 | 46

Molecules with partially positive and partially negative ends

Van der Waals attractions.

Hydrogen bonding and van der Waals attractions are nonbonding intermolecular interactions.

2 | 47

Boiling points of the normal alkanesIn Isomers

2 | 48

2,2-Dimethylpropane ball-and-stick model

2 | 49

Pentane: ball-and-stick model

2 | 50

2,2-Dimethylpropane space filling model

2 | 51

Pentane: space filling model

2 | 52

2,2-Dimethylpropane dash-wedge model

2 | 53

Pentane: dash-wedge model

3E. How to Name Alkyl Halides Rules

● Halogens are treated as substituents (as prefix)F: fluoro Br: bromoCl: chloro I: iodo

● Similar rules as alkyl substituents

2 | 54

Examples

Cl4 2

3 1

2-Bromo-1-chlorobutaneBr

1 32

4

1,4-Dichloro-3-methylhexaneCH3

Cl 56

Cl

2 | 55

3F. How to Name Alcohols IUPAC substitutive nomenclature:

a name may have as many as four features● Locants, prefixes, parent compound, and suffixes

OH5 3 16 4 2

4-Methyl-1-hexanol

2 | 56

Rules

● Select the longest continuous carbon chain to which the hydroxyl is directly attached. Change the name of the alkane corresponding to this chain by dropping the final –e and adding the suffix –ol

● Number the longest continuous carbon chain so as to give the carbon atom bearing the hydroxyl group the lower number. Indicate the position of the hydroxyl group by using this number as a locant

2 | 57

Examples

OH

2-Propanol(isopropyl alcohol)

32

1

45

3

4-Methyl-1-pentanol(or 4-Methylpentan-1-ol)

(NOT 2-Methyl-5-pentanol)

21

OH

OHOH

OH

1,2,3-Butanetriol

4 3 21

2 | 58

Example 4

OH

2 | 59

Example 4 (Cont’d)

● Find the longest chain as parent

Longest chain but does not contain the OH group

7-carbon chain containing the OH group

⇒ Heptane as parent

OH

12 3 4

56

7

OH

76

5 43

21

8

or

2 | 60

Example 4 (Cont’d)

● Use the lowest numbering for the carbon bearing the OH group

2(lowest number

of the carbon bearingthe OH group)

⇒ Use 2

6OH

76 5 4

32

1or

OH

12 3 4

56

7

2 | 61

Example 4 (Cont’d)

● Parent and suffix 2-Heptanol

● Substituents Propyl OH

12 3 4

56

7

OH

12 3 4

56

7

● Complete name 3-Propyl-2-heptanol

2 | 62

4. How to Name Cycloalkanes4A. How to Name Monocyclic

Cycloalkanes

Cycloalkanes with only one ring● Attach the prefix cyclo-

H2C CH2

CH2

=

Cyclopropane

=

Cyclopentane

CH2H2C

H2CCH2

CH2

2 | 63

Substituted cycloalkanes

Isopropylcyclopropane Methylcyclobutane

tert-Butylcyclopentane2 | 64

Example 1

1-Ethyl-3-methyl-cyclopentane

1

234

5

1-Ethyl-4-methyl-cyclopentane

1

543

2

NOT

3-Ethyl-1-methyl-cyclopentane

3

215

4

NOT

2 | 65

Example 2

4-Bromo-2-ethyl-1-methylcyclohexane1

2345

Br

6

1-Bromo-3-ethyl-4-methylcyclohexane4

3216

Br

5

NOT

(lowest numbers of substituents are 1,2,4 not 1,3,4)

2 | 66

Example 3

4-Ethyl-3-methylcyclohexanol6

123

4

OH5

(the carbon bearing the OH should have the lowest numbering, even though 1,2,4 is lower than 1,3,4)

1-Ethyl-2-methylcyclohexan-4-ol5

432

1

OH6

NOT

2 | 67

Cycloalkylalkanes● When a single ring system is attached to a single chain

with a greater number of carbon atoms

1-Cyclobutylpentane

● When more than one ring system is attached to a single chain

1,3-Dicyclohexylpropane

2 | 68

4B. How to Name Bicyclic Cycloalkanes

Bicycloalkanes● Alkanes containing two fused or bridged rings

Total # of carbons = 7● Bicycloheptane

Bridgehead

2 | 69

Example (Cont’d)

Between the two bridgeheads● Two-carbon bridge on the left● Two-carbon bridge on the right● One-carbon bridge in the middle

Complete name● Bicyclo[2.2.1]heptane

2 | 70

Other examples

7-Methylbicyclo[4.3.0]nonane

12

3

45

67

8

9

1-Isopropylbicyclo[2.2.2]octane

2

34

5

6

7

8

1

2 | 71

5. How to Name Alkenes &Cycloalkenes

Rule1. Select the longest chain that contains C=C as the parent

name and change the name ending of the alkane of identical length from –ane to–ene

2 | 72

Rule2. Number the chain so as to include both carbon atoms of

C=C, and begin numbering at the end of the chain nearer C=C. Assign the location of C=C by using the number of the first atom of C=C as the prefix. The locant for the alkene suffix may precede the parent name or be placed immediately before the suffix

2 | 73

● Examples

1-Butene(not 3-Butene)

CH2 CHCH2CH3

1 2 3 4

CH3CH CHCH2CH2CH3

2-Hexene(not 4-Hexene)

1 2 3 4 5 6

2 | 74

Rule3. Indicate the locations of the substituent groups by the

numbers of the carbon atoms to which they are attached ● Examples

2-Methyl-2-butene(not 3-Methyl-2-butene)

12

34

2 | 75

● Examples (Cont’d)

2,5-Dimethyl-2-hexene1

2

34

56

2,5-Dimethyl-4-hexene6

5

43

21NOT

2 | 76

Rule4. Number substituted cycloalkenes in the way that gives

the carbon atoms of C=C the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference

2 | 77

● Example

3,5-Dimethylcyclohexene

12

34

5

6

4,6-Dimethylcyclohexene

21

65

4

3NOT

2 | 78

Rule5. Name compounds containing a C=C and an alcohol

group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number

● Examples

2-Methyl-2-cyclohexen-1-ol(or 2-Methylcyclohex-2-en-1-ol)

12

34

5

6

OH

2 | 79

● Examples (Cont’d)

4-Methyl-3-penten-2-ol(or 4-Methylpent-3-en-2-ol)

12

34

5

OH

2 | 80

Rule6. Vinyl group & allyl group

Vinyl group

Ethenylcyclopropane(or Vinylcyclopropane)

ethenyl

Allyl group

prop-2-en-1-yl

OH

3-(Prop-2-en-1-yl)cyclohexan-1-ol

(or 3-Allylcyclohexanol)

1 2

34

5

6

2 | 81

Rule7. Cis vs. Trans

● Cis: two identical or substantial groups on the same side of C=C

● Trans: two identical or substantial groups on the opposite side of C=C

cis-1,2-DichloroetheneCl Cl Cl

Cl

trans-1,2-Dichloroethene

2 | 82

Example

2 | 83

Example (Cont’d)

12

34

5

67

12

34

56

57 12

34

6

31 76

54

2

(a)

(d)(c)

(b)

2 | 84

Example (Cont’d)● Complete name

31 76

54

2

4-tert-Butyl-2-methyl-1-heptene

2 | 85

6. How to Name Alkynes

Alkynes are named in much the same way as alkenes, but ending name with –yne instead of –ene

Examples

57

12346

2-Heptyne

3

1 4

2

4-Bromo-1-butyne

Br

2 | 86

Examples (Cont’d)

I Br

1

2 3 4

5 6 7 8 910

9-Bromo-7-iodo-6-isopropyl-8-methyl-3-decyne

2 | 87

OH group has priority over C≡C

1

234

3-Butyn-1-ol

OH

41 8

6

2-Methyl-5-octyn-2-ol

OH

3

5

27

4

321OHNOT

58 1

3OH

6

4

72

NOT

2 | 88

7. Physical Properties of Alkanes & Cycloalkanes

Boiling points & melting points

2 | 89

C6H14 Isomer Boiling Point (oC)

68.7

63.3

60.3

58

49.7

Physical Constants of Cycloalkanes

# of C Atoms Name bp (oC)

mp (oC)

Density

Refractive Index

3 Cyclopropane -33 -126.6 - -

4 Cyclobutane 13 -90 - 1.4260

5 Cyclopentane 49 -94 0.751 1.4064

6 Cyclohexane 81 6.5 0.779 1.4266

7 Cycloheptane 118.5 -12 0.811 1.4449

8 Cyclooctane 149 13.5 0.834 -

8. Sigma Bonds & Bond Rotation Two groups bonded by a single bond can undergo rotation

about that bond with respect to each other● Conformations – temporary molecular shapes

that result from a rotation about a single bond● Conformer – each possible structure of

conformation● Conformational analysis – analysis of energy

changes that occur as a molecule undergoes rotations about single bonds

2 | 92

8A. Newman Projections & How toDraw Them

H

OHClEt

HMe

Look from thisdirection

Sawhorse formula

H

Cl Et OH

Me H

OH

Me HH

EtClfront carbon back carbon

Newman Projection

combine

2 | 93

Look from thisdirection

Hc

H HbHa

HH

staggered conformationof ethane

f1 = 60o

f2 = 180o

8B. How to Do a Conformational Analysis

2 | 94

Look from thisdirection

eclipsed conformationof ethaneH H

H H

HHf = 0o

2 | 95

2 | 96

9. Conformational Analysis ofButane

Sawhorse formula Newman Projectionformula

Me

H HMe

HH

Me

MeHH

HH

2 | 97

2 | 98

CH3

H

CH3

HCH3

HH

H

CH3

HH

HCH3

H HCH3 H

H

anti conformer(I)

(lowest energy)

eclipsed conformer(II )

gauche conformer(III )

CH3

H HH H

H3C

eclipsed conformer(IV)

(highest energy)

CH3

H HH CH3

H

eclipsed conformer(VI)

H

CH3

HH

CH3H

gauche conformer(V)

Front carbon groups

rotate 60o clockwise

=

2 | 99

2 | 100

CH3

CH3

anti

CH3CH3

gauche

CH3CH3

eclipsed

0o

180o

60o

2 | 101

10. The Relative Stabilities ofCycloalkanes: Ring Strain

Cycloalkanes do not have the same relative stability due to ring strain

Ring strain comprises:

● Angle strain – result of deviation from ideal bond angles caused by inherent structural constraints

● Torsional strain – result of dispersion forces that cannot be relieved due to restricted conformational mobility

2 | 102

10A. Cyclopropane

H H

H H

H H

sp3 hybridized carbon(normal tetrahedral bond angle is 109.5o)

Internal bond angle (q) ~60o (~49.5o deviated from the ideal tetrahedral angle)

q

2 | 103

2 | 104

10B. Cyclobutane

H H

HH

H

H

HH

Internal bond angle (q) ~88o (~21o deviated from the normal 109.5o tetrahedral angle)

q

2 | 105

Cyclobutane ring is not planar but is slightly folded.

If cyclobutane ring were planar, the angle strain would be somewhat less (the internal angles would be 90o instead of 88o), but torsional strain would be considerably larger because all eight C–H bonds would be eclipsed

2 | 106

10C. Cyclopentane

H

H

H

H

H HHH H

H

If cyclopentane were planar, q ~108o, very close to the normal tetrahedral angle of 109.5o

However, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed)

Therefore cyclopentane has a slightly bent conformation

2 | 107

11. Conformations of Cyclohexane:The Chair & the Boat

12

3

45

6

1

2 3

456

(chair form)

(more stable)

(boat form)

(less stable)

3D

H

HH

HH

HH

H1

4

5 6 23

H

H

H

H

H

H

H

H1

45

6 23

2 | 108

The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to● Eclipsed conformation● 1,4-flagpole interactions

1 4

(eclipsed)

H H

H H

H H

2 | 109

The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation

(twist boat)

2 | 110

Energy diagram

2 | 111

12. Substituted Cyclohexanes: Axial& Equatorial Hydrogen Groups

The six-membered ring is the most common ring found among nature’s organic molecules

The chair conformation of a cyclohexane ring has two distinct orientations for the bonds that project from the ring: axial and equatorial

H

HH

H

HH

H

H

H

HH

H

2 | 112

12A. How to Draw Chair Conformational Structures

When you draw chair conformational structures, try to make the corresponding bonds parallel in your drawings

2 | 113

Axial hydrogen atoms in chair form• The axial bonds are all either up or down, in a vertical

orientation

H

H

H

H

H

H

2 | 114

Equatorial hydrogen atoms in chair form• The equatorial bonds are all angled slightly

HH

HH

HH

2 | 115

12B. A Conformational Analysis of Methylcyclohexane

Substituted cyclohexane• Two different chair forms

H

G

G

H

(equatorial G)(more stable)

(axial G)(less stable)

H

G

HG

(same as)

2 | 116

G

H

1,3-diaxial interaction

HH

13

3

The chair conformation with axial G is less stable due to 1,3-diaxial interaction

The larger the G group, the more severe the 1,3-diaxial interaction and shifting of the equilibrium from the axial-G chair form to the equatorial-G chair form

2 | 117

G

G(equatorial) (axial)

At 25oC

G% of

Equatorial% of Axial

F 60 40

CH3 95 5iPr 97 3tBu > 99.99 < 0.01

2 | 118

12C. 1,3-Diaxial Interactions of a tert-Butyl Group

The chair conformation with axial tert-butyl group is less stable due to 1,3-diaxial interaction

H

HH

13

H3CCH3

CH3

3

1,3-diaxial interaction

2 | 119

13. Disubstituted Cycloalkanes:Cis-Trans Isomerism

cis-1,2-Dimethylcyclopropane

CH3

H

CH3

H

trans-1,2-Dimethylcyclopropane

CH3

H CH3

H

Cl

H H

Cl Cl

H Cl

H

cis-1,2-Dichlorocyclobutane

trans-1,2-Dichlorocyclobutane

2 | 120

13A. Cis-Trans Isomerism and Conformation Structures of Cyclohexanes

Trans-1,4-Disubstituted Cyclohexanes

H

HCH3

H

CH3 H

H3CCH3

ring

flip

trans-Diaxial trans-Diequatorial

2 | 121

CH3H3C

H

H

trans-Dimethylcyclohexane

Upper bond

Lower bond

Upper-lower bonds means the groups are trans

2 | 122

Cis-1,4-Disubstituted Cyclohexanes

H

HH

H3C

CH3 CH3

HCH3

ring

flip

Equatorial-axial Axial-equatorial

chair-chair

2 | 123

CH3

CH3

ring

flipH3C

CH3H3C

H3CH3C CH3

(more stablebecause largegroup isequatorial)

(less stablebecause largegroup isaxial)

Cis-1-tert-Butyl-4-methylcyclohexane

2 | 124

Trans-1,3-Disubstituted Cyclohexanes

H

H3C

CH3

Hring

flip

trans-1,3-Dimethylcyclohexane

CH3

H

H

CH3

(eq)

(ax)

(ax)

(eq)

2 | 125

CH3

ring

flipH3C

CH3H3C

H3CH3C CH3

(more stablebecause largegroup isequatorial)

(less stablebecause largegroup isaxial)

CH3

Trans-1-tert-Butyl-3-methylcyclohexane

2 | 126

Cis-1,3-Disubstituted Cyclohexanes

ring

flip

(more stable)

CH3

H

CH3H

CH3 CH3

H H

(less stable)

2 | 127

Trans-1,2-Disubstituted Cyclohexanes

ring

flip

trans-1,2-Dimethylcyclohexane

CH3

CH3(eq)

(ax)

(ax)

(eq)

CH3

CH3

diequatorial(much more stable)

diaxial(much less stable)

2 | 128

CH3

ring

flipCH3

CH3CH3

cis-1,2-Dimethylcyclohexane(equal energy and equallypopulated conformations)

(equatorial-axial) (axial-equatorial)

(eq)

(ax)

(eq)

(ax)

Cis-1,2-Disubstituted Cyclohexane

2 | 129

14. Bicyclic & Polycyclic Alkanes

Decalin(Bicyclo[4.4.0]decane)

cis-Decalin trans-Decalin

H

H

H

H

HH

H

H 2 | 130

Adamantane Cubane Prismane

C60 (Buckminsterfullerene) 2 | 131

15. Chemical Reactions of Alkanes Alkanes, as a class, are characterized by a general inertness to

many chemical reagents

Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures

2 | 132

Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized

This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins (parum affinis, Latin: little affinity)

2 | 133

16. Synthesis of Alkanes andCycloalkanes

16A. Hydrogenation of Alkenes & Alkynes

C C

H2Pt, Pd or Ni

solventheat and pressure

C C

2H2Pt, Pd or Ni

solventheat and pressure

H H

HH

H H2 | 134

Examples

+ H2

Ni

EtOH

25oC, 50 atm.H H

Pd

EtOH

25oC, 1 atm.

+ H2

H

H

Pd

EtOAc

65oC, 1 atm.

H H

H H+ 2 H2

2 | 135

17. How to Gain Structural Inform-ation from Molecular Formulas & Index of Hydrogen Deficiency

Index of hydrogen deficiency (IHD)● 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

● Also known as “degree of unsaturation” or “double-bond equivalence” (DBE)

2 | 136

Index of hydrogen deficiency (Cont’d)

● Saturated acyclic alkanes: CnH2n+2

● Each double bond or ring: 2 hydrogens less

● Each double bond or ring provides one unit of hydrogen deficiency

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e.g.

and

1-Hexene Cycloheane

Hexane: C6H14

Index of hydrogendeficiency (IHD) =

– C6H12

C6H14

H2

= one pair of H2

= 1

C6H12

2 | 138

Examples

IHD = 2 IHD = 3

IHD = 2 IHD = 4

2 | 139

17A. Compounds Containing Halogen,Oxygen, or Nitrogen

For compounds containing● Halogen – count halogen atoms as

though they were hydrogen atoms● Oxygen – ignore oxygen atoms and

calculate IHD from the remainder of the formula

● Nitrogen – subtract one hydrogen for each nitrogen atom and ignore nitrogen atoms

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Example 1: IHD of C4H6Cl2

● Count Cl as H C4H6Cl2 ⇒ C4H8

● A C4 acyclic alkane:

C4H2(4)+2 = C4H10

IHD of C4H6Cl2 =

– C4H8

C4H10

H2

one pair of H2 = 1

● Possible structures

ClCl

Cl

Cl

Cl... etc.

or or

Cl2 | 141

Example 2: IHD of C5H8O● Ignore oxygen

C5H8O ⇒ C5H8

● A C5 acyclic alkane:

C5H2(5)+2 = C5H12

IHD of C4H6Cl2 =

– C5H8

C5H12

H4

two pairs of H2 = 2

● Possible structures

... etc.

or orOHO

OH

2 | 142

Example 3: IHD of C5H7N● Subtract 1 H for each N

C5H7N ⇒ C5H6

● A C5 acyclic alkane:

C5H2(5)+2 = C5H12

IHD of C4H6Cl2 =

– C5H6

C5H12

H6

three pair of H2 = 3

● Possible structures

C ... etc.orNCH3

N

2 | 143

2 | 144

2 | 145

Chlorination of hydrocarbons is a substitution reaction in which a chlorine atom is substituted for a hydrogen atom. Likewise in bromination reactions, a bromine atom is substituted for a hydrogen atom.

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2 | 147