Chapter 2 Lecture 2 Alkanes

I. Functional GroupsA. Hydrocarbons: organic molecules containing only C and H

1) Alkanes: saturated hydrocarbons

a) Saturated = all single bonds

b) Unsaturated = one or more double or triple bonds

2) Alkenes: hydrocarbons with one or more double bonds

3) Alkynes: hydrocarbons with one or more triple bonds

4) Aromatics: hydrocarbons containing

B. Other functional groups

1) Table 2-1 p. 54/55

2) Become familiar with these

II. AlkanesA. Structure

1) Straight-chain (normal) Alkanes n-hexane

2) Branched Alkanes

a) Straight-chain and Branched alkanes are constitutional isomers

b) CnH2n+2 = general formula

3) Cycloalkanes

a) General formula = CnH2n

b) Not constitutional isomers with straight or branched

4) Homologous Series: group of molecules differing only by a -CH2- group

5) Alkyl Group: fragment that is an alkane missing one H

1) R (radical or Residue) stands for any Alkyl Group

2) Methyl Ethyl

CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 CH3CH2CH2CH2CH3 CH3(CH2)nCH3

CH3 CH2CH3

B. Nomenclature: systematic naming by IUPAC rules

1) Straight Chain Alkanes

a) C1 = Methane C2 = Ethane C3 = Propane C4 = Butane

b) C5-C10 = pent-, hex-, hept-, oct-, non-, dec- -ane

c) C11-C19 = un-, do-, tri-, tetra-, penta-….nona -decane

d) C20 = Icosane

2) Branched Alkanes

a) Primary(1o) Secondary(2o) Tertiary(3o) Quaternary(4o)

Tell how many other carbon atoms are connected to the subject carbon

b) Substituents = groups attached to the main chain of an alkane

1o

2o

3o

4o

c) Rules for naming Branched Alkanes (or drawing structure from name)

1. Find the longest chain and name it as a straight chain alkane

2. Name substituents as alkyl groups

3. Number the main chain starting from the end closest to a substituent

4. Write the name

i. Alphabetize by sub. (di-, tri-count only if part of sub. name)

ii. Order #’s from low to high; use smallest possible numbers

iii. Capitalize the first letter only

iv. Write as one word with commas and hyphens as needed

v. Complex substituents in parentheses

5-Ethyl-2,2-dimethyloctane 4-ethyl-3,6-dimethyldecane

5-(1,1-Dimethylethyl)-3-ethyloctane

11’

4-(1-Ethylpropyl)-2,3-dimethylnonane

1

1

1

C. Properties of Alkanes

1) Structure

a) Very regular because bonds (C-C, C-H) are all sp3

b) Each C is tetrahedral

c) n-pentane

2) Physical Properties

a) Very predictable along homologous series

H

H

HH

H

H

H

HH

H H

b) Forces between Alkane Moleculesi. Ionic Solids: Ionic Forces (Coulombic) Na+Cl-

ii. Polar Solids: Dipole-Dipole Forces CH3Cliii. Nonpolar Solids: London Forces = weak attractive force due

to electron cloud correlation

c) Melting Points of Alkanesi. Very low, mostly liquids at room temperatureii. Increases with size (more area for London Forces)

d) Boiling Points of Alkanes: similar trends as m.p.

e) Branched Alkanes have lower m.p., b.p than linear alkanes: less surface area means less London Forces

D. Conformations of Alkanes

1) C-C Bonds can freely rotate (~3 kcal/mol energy barrier due to e- repul.)

2) Newman Projections

3) Potential Energy Diagrams for Rotation of Alkane Conformationsa) Torsional Angle = degrees of rotation in a conformational changeb) Transition State = energy maxima; transition from one lower

energy state to anotherc) Activation Energy = energy barrier to be overcome in changing

conformations

4) Conformational Analysis of Substituted Ethanes

5) Steric Hindrance = 2 fragments can’t occupy same space at same time

6) Butane: anti and gauche conformations