chapter 2 alkanes and cycloalkanes; conformational and...

Dr.Abdulaziz Ajlouni

Chapter 2

Alkanes and Cycloalkanes;Conformational and Geometrical Isomerism


Alkanes are hydrocarbons containing only single Bonds saturatedGeneral formula: CnH2n+2


Drawing chemical structures Several shorthand methods have been developed to write structures.Condensed structures don’t have C-H or C-C single bonds shown

• Straight-chain alkane: An alkane that has all its carbons connected in a row.

• Branched-chain alkane: An alkane that has a branching connection of carbons.

They are also called aliphatic compounds


Constitutional Isomers• Alkanes are saturated with hydrogen (no more can be

added• They are also called aliphatic compounds• Alkanes with C’s connected to no more than 2 other C’s

are straight-chain or normal alkanes• Alkanes with one or more C’s connected to 3 or 4 C’s are

branched-chain alkanes


Constitutional (Structural) Isomers • Isomers that differ in how their atoms are arranged in chains are

called constitutional isomers• Compounds other than alkanes can be constitutional isomers of

one another• They must have the same molecular formula to be isomers


• Alkyl group (R) : The part of an alkane that remains when a hydrogen atom is removed.


Types of Alkyl groups


Different Kinds of Carbons and Hydrogens


Nomenclature of AlkanesThe Name has Prefix+Parent + Suffix1. Determine the number of carbons in the longest continuouschain


2. Number the chain so that the substituent gets the lowest number


3. Number the substituents to yield the lowest possible number in the number of the compound

substituents are listed in alphabetical order


4. Assign the lowest possible numbers to all of the substituents


5. If the same substituent numbers are obtained in both directions,the first group cited receives the lower number

6. If a compound has two or more chains of the same length, theparent hydrocarbon is the chain with the greatest number ofsubstituents


Boiling Points of AlkanesBranched alkanes have less surface area contact,so weaker intermolecular forces Less boiling points.


Arrange the following compounds in order of decreasing their boiling points ?

1 2 3 4

A) 1< 2 < 3<4 B) 1<4< 3 < 2 C) 2 < 3 < 1<4 D) 4<1 < 3 < 2


Conformations of Alkanes:

Rotation about Carbon–Carbon Bonds

Conformers are Stereoisomers


Conformations of Alkanes

A staggered conformer is more stable than an eclipsed conformer


Different Conformations of Ethane


Conformations of n-Butane


Steric strain: repulsion between the electron clouds of atoms Or groupsThe gauche interaction destabilizes the gauche conformation


Nomenclature of Cycloalkanes1. No number is needed for a single substituent on a ring

2. Name the two substituents in alphabetical order


3. If there are more than two substituents, they are cited inalphabetical order


Cycloalkane Stability

• 5- and 6-membered rings most stable• Bond angle closest to 109.5°• Angle (Baeyer) strain• Measured by heats of combustion

per -CH2 -


Heats of Combustion/CH2Alkane + O2 → CO2 + H2O

Long-chain

658.6 kJ 658.6697.1 686.1

664.0 663.6 kJ/mol662.4


Cyclopropane

• Large ring strain due to angle compression• Very reactive, weak bonds

=>


Cyclobutane

• Angle strain due to compression• Torsional strain partially relieved by

ring-puckering

=>


Cyclopentane• If planar, angles would be 108°, but all

hydrogens would be eclipsed.• Puckered conformer reduces torsional strain.

=>


Cyclohexane

• Combustion data shows it’s unstrained.• Angles would be 120°, if planar.• The chair conformer has 109.5° bond

angles and all hydrogens are staggered.• No angle strain and no torsional strain.


Q Which of the following correctly ranks the cycloalkanes in order of increasingring strain per methylene?

• A) cyclopropane < cyclobutane < cyclohexane < cyclopentane

• B) cyclohexane < cyclopentane < cyclobutane < cyclopropane

• C) cyclohexane < cyclobutane < cyclopentane < cyclopropane•• D) cyclopentane < cyclopropane < cyclobutane < cyclohexane


Chair Conformer

=>


Axial and Equatorial Positions

=>


Steric Strain of 1,3-Diaxial Interaction in Methylcyclohexane

The larger the substituent on a cyclohexane ring, the more the equatorial substituted conformer will be favored


Ring Flipping in Cyclohexane


1,3-Diaxial Interactions

=>


Boat Conformer

=>


Cis-Trans Isomerism

• Cis: like groups on same side of ring• Trans: like groups on opposite sides of ring


The correct IUPAC name for

A) 1,3,3-trimethylcyclobutane. B) cis-1,1,3-trimethylcyclobutan C) trans-1,1,3trimethylcyclobutane. D) 1,1,3-trimethylcyclobutane

CH3

H

CH3

H3C

.

.


Cis-trans Isomerism of Di-substituted cyclohexane

1,2 disubstituted-Trans is diax or dieq (most stable)

-Cis is one is ax and one is eq1,3 disubstituted

-Trans is one is ax and one is eq-Cis is diax or dieq (most stable)

1,4 disubstituted (as 1,2)


The Chair Conformers of cis- or trans-1,4-Dimethylcyclohexane


In the most stable chair conformation of trans-1,2-dimethylcyclohexane, how many axial positions are occupied by hydrogen atoms?

H

H

H

H

H

H

6H

H

HH

H

H

H


The relationships of the various types of isomers.


What is the relationsip between the two structure?

• A) Stuctural isomers B) Conformers • C) Different compounds D) Confugrational isomers

H

CH3

H

CH3

CH3

H

H

H3C

H

CH2CH3

HH


Reactions of Alkanes

• Oxidation and Combustion

CH3CH2CH2CH3 + O2 CO2 + H2Oheat8 10132

CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +heat or light

• Halogenation


How many monobromo products can be obtained from the bromination of

12

3

chapter 2 alkanes and cycloalkanes; conformational and...

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