Download Alkanes File

Alkanes
L.O. 1. To know the general formula of alkanes.
2. To be able to construct simple mechanisms for
alkane reactions.
3. To find out how alkanes are extracted from
crude oil for use as fuels.
Starter: Name your compound, draw its full structural
formula and skeletal formula, and identify all functional
groups present
Alkanes – some key facts
Alkanes
CnH2n+2
(n = 1, 2, 3 etc)
Alkanes are generally very
because the molecules are
since C and H have
Alkanes are also described as
because they contain
Types of bond breaking
Homolytic bond breaking
Alkane
s
Homo – same
Covalent bond breaks equally
resulting in the production of
FREE RADICALS
Homolytic fission
Heterolytic bond breaking Hetero – different
Covalent bond breaks and one
atom takes both electrons.
Ionic intermediates result
Heterolytic fission
Alkanes with halogens, Cl2 or Br2
Initiated by energy from UV light
 successive substitution (replacement) of H
atoms in the alkane by Cl / Br atoms.
 mixture of chloro- / bromoalkanes
eg
Methane + Cl2
 mono-, di-, tri- and tetrachloromethanes
Excess CH4
 CH3Cl MORE likely
Excess Cl2
 CCl4 MORE likely
Halogen-methane substitution : mechanism
= a FREE RADICAL SUBSTITUTION mechanism
Also a CHAIN REACTION  a product from one
stage of process is
used in the next stage.
THREE stages : 1. INITIATION
(Start)
2. PROPOGATION
(Products Grow)
3. TERMINATION
(Finish)
1. Initiation
Caused by energy input from UV light
 covalent bond in a few Cl2 molecules broken
 HOMOLYSIS - one e- from bond pair to each atom
 neutral halogen atoms = FREE RADICALS
Cl
Cl
 Cl + Cl
UV energy
2. Propogation steps
.
(a) Cl free radical from initiation step collides
with a molecule of CH4 and removes a H atom.
H
H
C
H
H
H
+
Cl
H
C
+
H
 first substitution product (HCl)
.
+ a methyl free radical (H C ).
3
H Cl
(b)
.
CH free radical from step (a) collides with a
3
NEW molecule of Cl2 and removes a Cl atom.
H
H
C
H
H
Cl Cl
H
C
Cl
+
H
 second substitution product (CH3Cl)
.
+ a NEW chlorine free radical (Cl ).
Cl
Propagation steps (a) and (b) ALTERNATE
 HCl + CH3Cl each repetition of (a) and (b)
3. Termination steps

propagation steps stopped by
consumption of free radicals

combine to form neutral molecules
Free Radicals in
Mixture
CH3
Cl
.
.
Possible termination
products
CH3Cl
Cl2
CH3CH3
H
H
C
H +
U.V.
Cl2
H
H
+ Cl2
Cl
C
Cl
Cl
+
HCl
H chloromethane
H
Cl
Cl
C
+
H
H
HCl
Cl
+ HCl
Cl dichloromethane
tetrachloromethane
+ Cl2
Similarly for Br2
C
+ Cl2
Cl
H
C
Cl
Cl
+
HCl
trichloromethane
Your turn….
Draw out a full mechanism to show the free radical
substitution reaction of bromine with ethane to form
1,1-dibromoethane.
Reacting particles
1. Free radicals :
.
Neutral particle with an
unpaired electron
.
eg Cl , H3C
2. Nucleophiles :
“Nucleus loving” particle
- electron rich
- can provide a lone pair to
form a covalent bond
eg :NH3,:OH2, :OH-, :CN3. Electrophiles :
(NOT metal ions!)
“Electron loving” particle
- electron deficient
- can accept a lone pair to
form a covalent bond
eg H+, Br+, +CH3
Can you…….?
1. Confidently name organic compounds?
2. Confidently identify and name functional groups?
3. Confidently identify types of structural isomerism in
simple organic compounds?
4. Identify a free radical?
5. Draw the free radical substitution mechanism for the
halogenation of alkanes?
Homework: Prepare an A4 poster explaining how fuels (alkane fuels) are made
from crude oil.
Your poster must contain information on the following:
Fractional distillation, Cracking, Catalytic cracking, Reforming,
Adding ethanol and ethers
Due: Thursday 14th Jan