Structure of hydrocarbons

• Hydrocarbon:Hydrocarbon: a compound composed only of carbon and hydrogen

• Saturated hydrocarbon:Saturated hydrocarbon: a hydrocarbon containing only single bonds

• Alkane:Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain

• Aliphatic hydrocarbon:Aliphatic hydrocarbon: another name for an alkane

Hydrocarbons

H-C C-HH-C-C-H

H

H

H

H HC C

H

H H

Hydrocarbons

Alkanes(Chapter 2)

Alkenes(Chapters 5-6)

Alkynes(Chapter 7)

Arenes(Chapter 21-22)

Only carbon-carbon single

bonds

One or more carbon-carbondouble bonds

One or morecarbon-carbontriple bonds

One or morebenzenelike

rings

Class

Example

Carbon-carbon

bonding

Name Ethane Ethene Acetylene Benzene

Saturated Unsaturated

Structure

• Shape– tetrahedral about carbon– all bond angles are approximately 109.5°

Drawing Alkanes

• Line-angle formulas– an abbreviated way to draw structural

formulas– each vertex and line ending represents a

carbon

CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3

PentaneButanePropane PentanePropane

Structuralformula

Line-angle formula

Ball-and-stickmodel

Constitutional Isomerism

• Constitutional isomers:Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms– example: C4H10

CH3CH2CH2CH3

Butane(bp -0.5°C)

CH3

CH3CHCH3

2-Methylpropane(bp -11.6°C)

Constitutional Isomerism– do these formulas represent constitutional

isomers?

– find the longest carbon chain– number each chain from the end nearest the

first branch – compare chain lengths as well the identity and

location of branches

CH3CHCH2CHCH3 CH3

CH3

CH3CH2CHCHCH3

CH3

CH3

and (each is C7H16)

CH3CH2CHCHCH3

CH3

CH3

CH3CHCH2CHCH3

CH3

CH3

4

454

31and

5

1 12 23

3

52

5 4 3

2 1

Constitutional Isomerism

World populationWorld populationis aboutis about6,000,000,0006,000,000,000

4,111,846,763

4,347

75

31

Constitutional Isomers

MolecularFormula

CH4

C5H12

C10H22

C15H32

C30H62

36,797,588C25H52

Nomenclature - IUPAC

• Suffix -aneane specifies an alkaneane

• Prefix tells the number of carbon atoms

undec-dodec-

tetradec-pentadec-hexadec-heptadec-

nonadec-eicos-

tridec-

11121314151617

octadec- 181920

Prefixmeth-eth-prop-but-pent-hex-

oct-non-dec-

1234567hept-89

10

Carbons CarbonsPrefix

Nomenclature - IUPAC

• Parent name:Parent name: the longest carbon chain

• Substituent:Substituent: a group bonded to the parent chain – alkyl groupalkyl group:: a substituent derived by removal

of a hydrogen from an alkane; given the symbol R-

Alkane Alkyl group

CH4

Name Name

Methane CH3- Methyl group

CH3CH3 Ethane CH3CH2- Ethyl group

Nomenclature - IUPAC1.The name of a saturated hydrocarbon with an unbranched chain

consists of a prefix and suffix

2. The parent chain is the longest chain of carbon atoms

3. Each substituent is given a name and a number

4. If there is one substituent, number the chain from the end that gives it the lower number

CH3CHCH3

CH3

2-Methylpropane

12 3

CH3CH2CH2CHCH3

CH3

2-Methylpentane

123

45 5

432

1

(not 4-methylpentane)

Nomenclature - IUPAC

5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first

– indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc.

– use commas to separate position numbers

2,4-Dimethylhexane1

23

45

66

54

32

1

(not 3,5-dimethylhexane)

Nomenclature - IUPAC

6. If there are two or more different substituents,– list them in alphabetical order– number from the end of the chain that gives

the substituent encountered first the lower number

3-Ethyl-5-methylheptane

12

34

5 67 7

65

43 2

1

(not 3-methyl-5-ethylheptane)

Nomenclature - IUPAC

7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

– alphabetize the names of substituents first and then insert these prefixes

4-Ethyl-2,2-dimethylhexane(not 2,2-dimethyl-4-ethylhexane)

12 3

45

6

Nomenclature - IUPAC

• Alkyl groups

1-methylethyl (isopropyl)

propyl

ethyl

methyl

CondensedStructural FormulaName

CH3

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3

1,1-dimethylethyl (tert-butyl)

1-methylpropyl (sec-butyl)

2-methylpropyl (isobutyl)

butyl

CH3

CH3

CH3

CH3

-CH2CH2CH2 CH3

-CH2CHCH3

-CHCH2CH3

-CCH3

CondensedStructural FormulaName

Nomenclature - Common• The number of carbons in the alkane

determines the name– all alkanes with four carbons are butanes,

those with five carbons are pentanes, etc.– iso- indicates the chain terminates in -

CH(CH3)2; neo- that it terminates in -C(CH3)3

CH3CH2CH2CH2CH3

CH3CHCH3

CH3

CH3CH2CHCH3

CH3

CH3CH2CH2CH3

CH3CCH3

CH3

CH3

Pentane Isopentane

IsobutaneButane

Neopentane

Classification of C & H• Primary (1°) C:Primary (1°) C: a carbon bonded to one other

carbon– 1° H: a hydrogen bonded to a 1° carbon

• Secondary (2°) C:Secondary (2°) C: a carbon bonded to two other carbons– 2° H: a hydrogen bonded to a 2° carbon

• Tertiary (3°) C:Tertiary (3°) C: a carbon bonded to three other carbons– 3° H: a hydrogen bonded to a 3° carbon

• Quaternary (4°) CQuaternary (4°) C: a carbon bonded to four other carbons

Cycloalkanes• General formula CCnnHH2n2n

– five- and six-membered rings are the most common• Structure and nomenclature

– to name, prefix the name of the corresponding open-chain alkane with cyclo-cyclo-,, and name each substituent on the ring

– if only one substituent, no need to give it a number– if two substituents, number from the substituent of

lower alphabetical order– if three or more substituents, number to give them the

lowest set of numbers and then list substituents in alphabetical order

Cycloalkanes

• Line-angle drawings– each line represents a C-C bond– each vertex and line ending represents a C

C

C CC

CC

C

C H2C

H2CCH2

CH

CH2

CH

CH3

CH3

C8H16

Cycloalkanes

• Example:Example: name these cycloalkanes

(a) (b)

(c) (d)

IUPAC - General• prefix-infix-suffix

– prefixprefix tells the number of carbon atoms in the parent – infixinfix tells the nature of the carbon-carbon bonds– suffixsuffix tells the class of compound

one or more triple bonds

one or more double bonds

all single bonds

-yn-

-en--an-

Nature of Carbon-Carbon Bonds in the Parent Chain

InfixSuffix Class

-e

-ol

-al

-one

-oic acid

hydrocarbon

alcohol

aldehyde

ketone

carboxylic acid

-amine amine

IUPAC - Generalprop-enen-e = propene

eth-anan-ol = ethanol

but-anan-one = butanone

but-anan-al = butanal

pent-anan-oic acid = pentanoic acid

cyclohex-anan-ol = cyclohexanol

eth-ynyn-e = ethyne

eth-anan-amine = ethanamine

CH3CH=CH2

CH3CH2OH

CH3CH2CH2CH2COH

O

HC CH

OH

CH3CH2CH2CH

O

CH3CCH2CH3

O

CH3CH2NH2

Cis,Trans Isomerism

• Stereoisomers: Stereoisomers: compounds that have – the same molecular formula – the same connectivity – a different orientation of their atoms in space

• Cis,transCis,trans isomersisomers – stereoisomers that are the result of the

presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

Isomers

• relationships among isomers

Cis,Trans Isomers

• 1,2-Dimethylcyclopentane

CH3

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

H3CH3C CH3

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

Cis,Trans Isomerism

• 1,4-Dimethylcyclohexane

trans-1,4-Dimethyl-cyclohexane

cis-1,4-Dimethyl-cyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

CH3

H3C H3C

CH3

Cis,Trans Isomerism• trans-1,4-Dimethylcyclohexane

– the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol

CH3

H

H

CH3

HH3C

CH3

H(more stable)(less stable)

Cis,Trans Isomerism

• cis-1,4-Dimethylcyclohexane

conformations are of equal stability

H

CH3

H

CH3

H

H3C

H

CH3

Physical Properties

• Constitutional isomers have different physical properties

NameDensity (g/mL)

hexane2-methylpentane

3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.3

63.358.0

49.7

-95

-154

-118-129

-98

0.659

0.653

0.6640.661

0.649

bp (°C)

mp (°C)

Heats of Combustion• For constitutional isomers [kJ (kcal)/mol]

-5470.6 (-1307.5) -5465.6 (-1306.3) -5458.4 (1304.6)-5451.8 (1303.0)

8CO2 + 9H2O

Heat of Combustion

– strain in cycloalkane rings as determined by heats of combustion

Sources of Alkanes

• Natural gas – 90-95% methane

• Petroleum– gases (bp below 20°C)– naphthas, including gasoline (bp 20 - 200°C)– kerosene (bp 175 - 275°C)– fuel oil (bp 250 - 400°C)– lubricating oils (bp above 350°C)– asphalt (residue after distillation)

• Coal

Gasoline

• Octane rating:Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

Heptane(octane rating 0)

2,2,4-Trimethylpentane(octane rating 100)

Unsaturated Hydrocarbons• Unsaturated hydrocarbonUnsaturated hydrocarbon: contains one or

more carbon-carbon double or triple bonds

• AlkeneAlkene: contains a carbon-carbon double bond and has the general formula CnH2n

Ethene(an alkene)

H

C C

H

H H

Unsaturated Hydrocarbons• AlkyneAlkyne: contains a carbon-carbon triple

bond and has the general formula CnH2n-2

Ethyne(an alkyne)

H-C C-H

Unsaturated Hydrocarbons

• ArenesArenes: benzene and its derivatives

CC

CC

C

C

H

H

H

HH

H

Benzene(an arene)

Benzene & Phenyl Groupthe phenyl group is not reactive under any of the

conditions we describe for alkynes

Benzene Alternative representations for the phenyl group

C6H5- Ph-

Structure of Alkenes

• The two carbon atoms of a double bond and the four atoms attached to them lie in a plane, with bond angles of approximately 120°

• According to the orbital overlap model, a double bond consists of

• one sigma bond formed by overlap of sp2 hybrid orbitals

• one pi bond formed by overlap of parallel 2p orbitals

Structure of Alkenes• Length of C-C bonds: single > double >

triple

• Strength of C-C bonds:triple > double > single

sp-sp, two 2p-2p 1.21 200

1461.34sp2-sp2, 2p-2p

MoleculeC-C OrbitalOverlap Å kcal/mol

sp3-sp3 1.54 90ethane

ethylene

acetylene

Cis-Trans Isomerism

• Because of restricted rotation about a C-C double bond, groups on adjacent carbons are either cis or trans to each other

cis-2-Butenemp -139°C, bp 4°C

trans-2-Butenemp -106°C, bp 1°C

C

H3C

C

H

CH3

C

HH

C

CH3

HH3C

Nomenclature - Alkenes

• Use the infix -enen- to show the presence of a carbon-carbon double bond

• Number the parent chain to give the 1st carbon of the double bond the lower number

• Follow IUPAC rules for numbering and naming substituents

• For a cycloalkene, the double bond must be numbered 1,2

Nomenclature - Alkenes

6 5 4 3 2 1

4-Methyl-1-hexene

CH3 CH2CHCH2CH=CH2

CH3124 3

2-Ethyl-3-methyl-1-pentene

CH3 CH2CHC=CH25

CH3

CH2CH3

1 2

34

5

3-Methylcyclo-pentene

CH3

1,6-Dimethylcyclo-hexene

CH3

CH31

65

4

3

2

Nomenclature - Alkenes

allyl chloride

vinyl chloride

allyl

vinyl

ExampleCommonName

Alkenyl Group

CH2 =CH-

CH2 =CHCH2-

CH2 =CHCl

CH2 =CHCH2Cl

Nomenclature - Alkenes

• Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

IsobutylenePropyleneEthyleneCommon:

IUPAC: 2-MethylpropenePropeneEthene

CH2=CH2 CH3CH=CH2 CH3C=CH2

CH3

Nomenclature - Alkynes

• IUPAC: use the infix -ynyn- to show the presence of a carbon-carbon triple bond

1

3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne

234 1 2 3 4 5 6 7CH3CH2 C CCH2 CCH3

CH3

CH3CH3

CH3 CHC CH

Nomenclature - Alkynes

• Common names: prefix the substituents on the triple bond to the name “acetylene”

MethylacetyleneDimethylacetyleneVinylacetylenePropyne 2-Butyne 1-Buten-3-yne

CH3C CH CH3C CCH3 CH2=CHC CHIUPAC:

Common:

Configuration - cis, trans

• The cis-trans systemThe cis-trans system: configuration is determined by the orientation of atoms of the main chain

cis-3,4-Dimethyl-2-pentene

1

2 3

4C

H

C

CH3

CH(CH3)2H3C

trans-3-Hexene

C

H

C

CH2CH3

HCH3 CH2

Configuration - E,Z

• The E,Z system uses the priority rules of the R,S system to assign to the groups on each carbon of a carbon-carbon double bond1. Each atom bonded to the C-C double bond is

assigned a priority

2. If groups of higher priority are on the same side, configuration is Z (German, zusammen)

3. If groups of higher priority are on opposite sides, configuration is E (German, entgegen)

Configuration - E,Z• ExampleExample: name each alkene and specify its configuration by the E,Z

system

(a) (b)C

H3C CH(CH3 )2

C

CH3H

C

Cl

C

H

CH2 CH3H3C

(d)(c) C

Br CH3

C

HCl

C

ClCH2

C

CH3

CH2 CH3H3C

Physical Properties

• Alkenes are nonpolar compounds

• The only attractive forces between their molecules are dispersion forces

• The physical properties of alkenes are similar to those of alkanes

Terpenes

• TerpeneTerpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

CH2=C-CH=CH2

CH3

2-Methyl-1,3-butadiene(Isoprene)

C-C-C-C1 2 3 4head tail

Isoprene unit

C

Terpenes

Myrcene, C10H16, a component of bayberry wax and oils of bay and verbena

12

34

12

3

4

two isoprene units joined here

OH

•Menthol, from peppermint

O

Camphor

• Camphor, from the camphor tree

Vitamin A (Retinol)

H3C CH3

CH3

CH3 CH3

CH2OH

Benzene - Resonance Model• The concepts of hybridization of atomic

orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure and reactivity– the carbon skeleton is a regular hexagon– all C-C-C and H-C-C bond angles 120°

sp2-sp2

sp2-1s109 pm

120°

120°120°

139 pm

C

C

C

C

C CH

H H

H

H H

Benzene - The Resonance Model

• The pi system of benzene– (a) the carbon framework with the six 2p

orbitals– (b) overlap of the parallel 2p orbitals forms

one torus above the plane of the ring and another below it

– this orbital represents the lowest-lying pi-bonding molecular orbital

Benzene - Resonance

• We often represent benzene as a hybrid of two equivalent Kekulé structures– each makes an equal contribution to the

hybrid and thus the C-C bonds are neither double nor single, but something in between

Benzene as a hybrid of two equivalentcontributing structures

Benzene - Resonance

• Resonance energy:Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds– one way to estimate the resonance energy of

benzene is to compare the heats of hydrogenation of benzene and cyclohexene

Benzene

Concept of Aromaticity

• The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations

• To be aromatic, a compound must– be cyclic– have one p orbital on each atom of the ring– be planar or nearly planar so that there is continuous

or nearly continuous overlap of all p orbitals of the ring

– have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals

Heterocyclic Aromatics

• Heterocyclic compound:Heterocyclic compound: a compound that contains more than one kind of atom in a ring – in organic chemistry, the term refers to a ring

with one or more atoms are other than carbon

• Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

Pyridine

N

N

N••••

• •

Pyrimidine

12

3

4

5

6

12

3 4

5

6

Nomenclature of aromatic compounds

• Monosubstituted alkylbenzenes are named as derivatives of benzene– many common names are retained

Toluene CumeneEthylbenzene Styrene

Phenol Aniline Benzoic acid Anisole

COOHNH2 OCH3OH

Benzaldehyde

CHO

Nomenclature• Benzyl and phenyl groups

(Z)-2-Phenyl-2-butene

4-(3-Methoxyphenyl)-2-butanone

1-Phenyl-1pentanone

O OH3CO

Ph

Benzene Phenyl group, Ph- Toluene Benzyl group, Bn-

CH3 CH2-

Disubstituted Benzenes• Locate two groups by numbers or by the

locators orthoortho (1,2-), metameta (1,3-), and parapara (1,4-)– where one group imparts a special name, name

the compound as a derivative of that molecule

CH3

Br

COOHNO2

Cl

NH2

CH3

CH3

2-Nitrobenzoic acid

(o-Nitrobenzoic acid)

3-Chloroaniline(m-Chloroaniline)

4-Bromotoluene(p-Bromotoluene)

m-Xylene

Polysubstituted Derivatives– if one group imparts a special name, name the

molecule as a derivative of that compound– if no group imparts a special name, list them in

alphabetical order, giving them the lowest set of numbers

CH3

NO2

OHBr

Br

NO2

CH2CH3

Br

4

2

1

6

4

21

4

1 2

4-Chloro-2-nitro-toluene

2,4,6-Tribromo-phenol

2-Bromo-1-ethyl-4-nitrobenzene

Br

Cl

Phenols• The functional group of a phenol is an -OH

group bonded to a benzene ring

1,2-Benzenediol(Catechol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OHOHOH

OHCH3

OH

Phenols– hexylresorcinol is a mild antiseptic and

disinfectant– eugenol is used as a dental antiseptic and

analgesic– urushiol is the main component of the oil of

poison ivyOH

OH

OHOCH3 OH

OH

Hexylresorcinol Eugenol Urushiol

Quinones

• Important chemical property of quinones is that they are readily reduced to hydroquinones

1,4-Benzoquinone(p-Quinone)

(reduction)

1,4-Benzenediol(Hydroquinone)

O

O

OH

OH

Na2S2O4, H2O