2 | 1 Chapter 2: Alkanes and Cycloalkanes; Conformational and Geometric Isomerism
Feb 24, 2016
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 6NOT
7 5 36
(2,4,5-Trimethylheptane)
4 2 1
NOT
1 3 52
(3,4,6-Trimethylheptane)
4 6 7
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
2 3 4
5 6
7 8
9
9
8 7 6
5 4
3 2
1
instead of
(3,4,7) (3,6,7)2 | 27
Example 2 (Cont’d)
● Substituents 3,7-dimethyl 4-ethyl
1
2 3 4
5 6
7 8
9
2 | 28
Example 2 (Cont’d)
● Substituents in alphabetical order Ethyl before dimethyl
(recall Rule 4 – disregard “di”)● Complete name
1
2 3 4
5 6
7 8
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 23 1
2-Bromo-1-chlorobutaneBr
1 324
1,4-Dichloro-3-methylhexaneCH3
Cl 5 6Cl
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 24-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
ExamplesOH
2-Propanol(isopropyl alcohol)
3 2 1
453
4-Methyl-1-pentanol(or 4-Methylpentan-1-ol)
(NOT 2-Methyl-5-pentanol)
2 1OH
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
1 2 3 4 5 6 7
OH
76
5 4 3 2 1
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
1 2 3 4 5 6 7
OH
1 2 3 4 5 6 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 CH2CH2
=
Cyclopropane
=
Cyclopentane
CH2H2CH2C C
H2
CH2
2 | 63
Substituted cycloalkanes
Isopropylcyclopropane Methylcyclobutane
tert-Butylcyclopentane2 | 64
Example 1
1-Ethyl-3-methyl-cyclopentane1
2345
1-Ethyl-4-methyl-cyclopentane
1543
2
NOT
3-Ethyl-1-methyl-cyclopentane
3215
4
NOT
2 | 65
Example 2
4-Bromo-2-ethyl-1-methylcyclohexane1 2
345
Br
6
1-Bromo-3-ethyl-4-methylcyclohexane4 3
216
Br
5
NOT
(lowest numbers of substituents are 1,2,4 not 1,3,4)
2 | 66
Example 3
4-Ethyl-3-methylcyclohexanol6 1
234
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 4
321
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-Dicyclohexylpropane2 | 68
4B. How to Name Bicyclic Cycloalkanes
Bicycloalkanes● Alkanes containing two fused or bridged rings
Total # of carbons = 7● Bicycloheptane
Bridgehead2 | 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
1 23
45
678
9
1-Isopropylbicyclo[2.2.2]octane2
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 CHCH2CH31 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
23
45
6
2,5-Dimethyl-4-hexene6
54
32
1NOT
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)
1 23
45
6OH
2 | 79
● Examples (Cont’d)
4-Methyl-3-penten-2-ol(or 4-Methylpent-3-en-2-ol)
12345
OH
2 | 80
Rule6. Vinyl group & allyl group
Vinyl group
Ethenylcyclopropane(or Vinylcyclopropane)
ethenyl
Allyl group
prop-2-en-1-ylOH
3-(Prop-2-en-1-yl)cyclohexan-1-ol
(or 3-Allylcyclohexanol)
1 2
34
56
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)
1234 5
67
12345
6
57 12346
31 76542
(a)
(d)(c)
(b)
2 | 84
Example (Cont’d)● Complete name
31 76542
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
31 4
2
4-Bromo-1-butyne
Br
2 | 86
Examples (Cont’d)
I Br
1
2 3 45 6 7 8 9
10
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
OH3
52
7
4
321OHNOT
58 1
3OH6
47
2
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
HCl 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 groupsrotate 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 Hsp3 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
1 2 3
456
1
2 3
456
(chair form)(more stable)
(boat form)(less stable)
3D
H
HH
HH
HH
H1
45 6 2 3
H
H
H
H
H
H
H
H14
56 2
3
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 HH 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
HH
HH
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)
HG
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 40CH3 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
HH
H
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 HH3C
CH3
ringflip
trans-Diaxial trans-Diequatorial2 | 121
CH3H3CH
Htrans-Dimethylcyclohexane
Upper bond
Lower bond Upper-lower bonds means the groups are trans
2 | 122
Cis-1,4-Disubstituted Cyclohexanes
H
HHH3C
CH3 CH3H
CH3
ringflip
Equatorial-axial Axial-equatorial
chair-chair
2 | 123
CH3
CH3
ringflipH3C
CH3H3C
H3CH3C CH3
(more stablebecause largegroup isequatorial)
(less stablebecause largegroup isaxial)
Cis-1-tert-Butyl-4-methylcyclohexane
2 | 124
Trans-1,3-Disubstituted Cyclohexanes
HH3C
CH3Hring
flip
trans-1,3-Dimethylcyclohexane
CH3
HH
CH3
(eq)
(ax)
(ax)
(eq)
2 | 125
CH3
ringflipH3C
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
ringflip
(more stable)CH3
HCH3
H
CH3 CH3
H H
(less stable)
2 | 127
Trans-1,2-Disubstituted Cyclohexanes
ringflip
trans-1,2-Dimethylcyclohexane
CH3
CH3(eq)
(ax)
(ax)
(eq)
CH3
CH3diequatorial
(much more stable)diaxial
(much less stable)
2 | 128
CH3
ringflipCH3
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 CH2
Pt, Pd or Nisolvent
heat and pressure
C C2H2
Pt, Pd or Nisolvent
heat and pressure
H H
HH
H H2 | 134
Examples
+ H2Ni
EtOH25oC, 50 atm.
H H
PdEtOH
25oC, 1 atm.
+ H2
H
H
PdEtOAc
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
2 | 137
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
2 | 140
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
Cl Cl ClCl
Cl... etc.
or orCl
2 | 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 orOH
OOH
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
N2 | 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.
2 | 146
2 | 147