Alkanes

Saturated hydrocarbons

Structure of alkanes

Contain only C & HSingle bonds only – all carbons are sp3

and tetrahedralAll bond angles close to 109.5ºGeneral formula – CnH2n+2

Constitutional (structural) isomerismCompounds with the same empirical formula

but different structural formulas

Structure of alkanes

Straight chain (n-)

CH3CH2CH2CH3

n-butaneBranched

CH3

|

CH3CHCH3

i-butane (isobutane)

Nomenclature of alkanes

IUPAC systemPrefixes

1=meth 5=pent 8=oct

2=eth 6=hex 9=non

3=prop 7=hept 10=dec

4=but

CH3CH2CH2CH2CH3

pentane

Nomenclature of alkanes

Common names

isobutane isopentane

Nomenclature of alkanes

Endings-ane – all single bonds-ene – at least one double bond-yne – at least one triple bond

H2C=CHCH3

propene

HC≡CCH3

propyne

Nomenclature of alkanes

Branches are named after the number of carbons in the branch, + -yl-CH3 = methyl-CH2CH3 = ethyl-CH2CH2CH3 = propyl

Common names for substituents |

CH3CHCH3

Isopropyl (methylethyl)

Nomenclature of alkanes

|

CH3CHCH2CH3

sec-butyl (1-methylpropyl)

-CH2CHCH3

|

CH3

Isobutyl (2-methylpropyl)

Nomenclature of alkanes

|

CH3CCH3

|

CH3

tert-butyl

(1,1-dimethylethyl)

Nomenclature of alkanes

Numbering – the longest chain is numbered so that the first branch has the lowest possible number.

2-methylpentane

Nomenclature of alkanes

3-methylpentane Numbering is applied to the longest

possible chain

incorrect

Nomenclature of alkanes

3-methylhexaneMultiple identical substituents use prefixes

2 = di 3 = tri 4 = tetra

5 = penta 6 = hexa, etc.

Nomenclature of alkanes

Each substituent is designated by number

2, 4-dimethylpentane

Nomenclature of alkanes

Different substituents are named in alphabetical order

4-ethyl-2-methylhexaneCycloalkanesAdd “cyclo-“ in front of the chain name

Nomenclature of alkanes

cyclopropaneCan also be represented by a line-angle

drawing

Nomenclature of alkanes

cyclopropane cyclobutaneSubstituents are named as in other

alkanesDraw a structure for methylcyclohexane

Nomenclature of alkanes

methylcyclohexane 1,4-dimethylcyclohexane

1-ethyl-4-methylcyclohexane

Nomenclature of alkanes

When the ring has fewer carbons than the longest straight chain, the ring is named as a substituent.

2-cyclopropylbutane

Physical properties

Most alkanes are liquids or gases (C4 or below). Alkanes are nonpolar due to nonpolar nature or

C-H bond.Only intermolecular forces are weak London

dispersion forces (a type of van der Waals interaction) – dispersion forces increase with increasing molecular surface area

Boiling point increases with molar massBoiling point decreases with increased

branching

Physical properties

Solubility – insoluble in water or smaller alcohols, soluble in each other or any nonpolar solvent (carbon tetrachloride, benzene, ether)

Density – less than waterStrong odorInsulator

Conformations

Straight chain alkanesEthane

Newman projections

Conformations

Energy barrier to rotation is 2.8-3.0 kcal/mol.

At RT there is enough thermal energy to allow rapid rotation about the C-C bond.

1 kcal/mol for each H-H interaction

Conformations

Conformations

Butane conformations along the central C-H bond

Conformations

methyl-methyl interactions are 3kcal/molmethyl-H interactions are 1.25 kcal/mol

Conformations of butane

Conformations of cycloalkanes

Cyclopentane – envelope conformation – all bond angles 109.5º

CyclohexanesChair conformation – most stable

Conformations of cycloalkanes

Boat and twist conformations – less stable

twist

boat

Conformations of cycloalkanes

Ring positions – equatorial and axial

Conformations of cycloalkanes

Equatorial positions are less sterically hindered.

If large groups are attached to the ring they will most always be in the equatorial position.

Large groups in the axial position interfere with groups in the other axial positions.

Conformations of cycloalkanes

Cis-trans isomerism in cycloalkanes

Cyclobutanes and cyclopentanes

trans-1,2-dimethylcyclobutane

cis-1,2-dimethylcyclobutane

Cis-trans isomerism in cycloalkanes

Cyclohexanes

cis-1,2-dimethylcyclohexane

Cis-trans isomerism in cycloalkanes

Adjacent (1,2) cis substituents are equatorial-axial

cis-1,3 substituents are equatorial/equatorial

cis-1,4 substituents are equatorial-axial

Chemical reactions

Mostly unreactiveCombustion

2C6H14+19O2 12CO2+14H2O

Halogenation

2C3H8 + Cl2 2CH3CHClCH3

F2>Cl2>Br2>I2

3º>2º>1º

Sources of Alkanes

Petroleum – products of refining processNatural gas