Alkanes
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Transcript of Alkanes
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