By
Dr. Nahed Nasser
THE CHEMISTRY OF ALKANES
CONTENTS
>> Hydrocarbons
>>Structure of alkanes
>> Representation of M F
>> Classes of carbon and hydrogen atoms
>> Hybridisation of orbitals
>> Structural isomerism in alkanes
>>Alkyl Groups and Structural isomerism
>> IUPAC nomenclature of alkanes
>> Physical properties of alkanes
>> Preparation of alkanes
>> Reactions of alkanes
>> Pollution
>>Cycloalkanes, nomeclature , cis / trans isomerism and reactions
Alkanes : CnH2n+2
Name
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
3
Decane
Molecular Formula
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
REPRESENTATION OF
MOLECULAR FORMULAS
4
5
Drawing alkanes
n-Pentane
6
Classes of carbon and Hydrogen
•
•
•
•
7
Primary carbon : CH3-CH2-CH3
Secondary carbon : CH3-CH2-CH3
Tertiary carbon : (CH3)2-CH-CH3
Hydrogens are also referred to as 1º, 2º or
3º according to the type of carbon they are
bonded to.
Sp3 HYBRIDISATION OF ORBITALS
The electronic configuration of a
carbon atom is 1s22s22p2
2p
2
2s
1
If you provide a bit of energy you
can promote (lift) one of the s
electrons into a p orbital. The
configuration is now 1s22s12p3
2p
2
2s
1
8
1s
1s
The process is favourable because the arrangement of
electrons; four unpaired and with less repulsion is more stable
The four orbitals (an s and three p’s) combine or HYBRIDISE to give four
new orbitals. Because one s and three p orbitals are used, it is called sp3
hybridisation
All four orbitals are equivalent in energy, each with 25 % S character and
75 % P character.
2s22p2
9
2s12p3
4 x sp3
THE STRUCTURE OF ALKANES
In ALKANES, the four sp3
orbitals of carbon repel each
other into a TETRAHEDRAL
arrangement with bond angles
of 109.5º.
Each sp3 orbital in
carbon overlaps with
the 1s orbital of a
hydrogen atom to form
a C-H bond.
10
109.5º
11
The length of the bond: 1.54 A
Angle: 109.5
Structural Isomerism in alkanes
Different compounds with identical molecular formulas are
called ISOMERS and the phenomenon is called ISOMERISM
Boiling point
12
Melting
point
Butane
0
-138
Isobutane
-121
-145
Pentane, C5H12 has three chain isomers
CH3
H2
C
H3C
H2
C
C
H2
CH3
n-Pentane
H3C
H2C
C
H
2-Methyl-butane
Isopentane
CH3
CH3
H3C
C
CH3
CH3
2,2-Dimethyl-propane
Neopentane
Exercise :
Draw all possible structural isomers for the
M.F. C6 H14 . And name them Or
How many isomeric structures can be
exhibited by the M.F. C6 H14 ?
13
Alkyl groups
• Alkyl groups are formed by loss of a hydrogen
atom from the corresponding alkane
• ( e.g. CH4 Methane – 1 H = CH3 Methyl group )
• Alkyl groups are named by dropping the -ane
suffix of the alkanes and adding the suffix -yl.
Methane becomes a methyl group, ethane an
ethyl group, etc.
14
Structural isomerism in alkyl groups
• Propyl group C3H7 (can give two isomeric
alky groups)
OR
CH3
CH 3-CH2-CH 2n-Propyl
15
CH3 -CH
Isopropyl
Butyl group C4H9
It can exist in three isomeric forms
CH3
CH3
CH3-CH2-CH2-CH2-
CH3 -CH
H2
C
H3C
C
CH3
n-Butyl
16
Isobutyl
tert.Butyl
IUPAC NOMENCLATURE OF BRANCHEDCHAIN ALKANES
1-
Locate the longest continuous chain of carbon atoms; this chain
determines the parent name for the alkane.
Sometimes, you may need to go around corners and zigzag to find the
longest (parent) chain. (the parent chain is in blue):
CH3
CH3CH2CH2CH2CHCH3
CH3
CH3CH2CH2CH2CHCH3
CH2
H3C
H
C
H2C
CH
CH2
CH2
CH3
CH3
• If the parent chain for example has 6 carbon atoms, therefore, it is
a derivative of hexane and if it has 4 carbon atoms it is derivative of
butane and so on .
17
CH3
2- Number the longest chain beginning
with the end of the chain nearer to the
substituent.
Substituent
6 5 4 3 2 1
CH3CH2CH2CH2CHCH3
Substituent
CH3
7 6 5 4
3
CH3CH2CH2CH2CHCH3
2 CH2
1CH3
18
3- Use the numbers obtained by application
of rule 2 to designate the location of the
substituent group.
19
The parent name is placed last; the
substituent group, preceded by the
number indicating its location on the chain,
is placed first.
4. When two or more substituents are
present, give each substituent a number
corresponding to its location on the
longest chain.
The substituent groups are listed alphabetically
regardless of their order of occurrence in the molecule.
Cl is called chloro, Br called bromo, I called iodo, NO2
20
called
nitro, CN called cyano
5) When two or more substituents are
identical, indicate this by the use of the
prefixes di-, tri-, tetra-, and so on.
In case of deciding alphabetical order of many
substituent disregard multiplying prefixes such as
“di”and “tri”, “tetra”, “penta”, ….
21
6) When two substituents are present on the
same carbon, use the number twice.
CH3
H3CCH2
C
CH2CH2CH3
CH2
CH3
3-Ethyl-3-methylhexane
22
7. When two chains of equal length compete for
selection as the parent chain, choose the
chain with the greater number of
substituents.
23
8. When branching occurs at an equal
distance from both ends of the longest
chain, choose the name that gives the
lower number at the first point of
difference.
24
1.
2.
3.
4.
5.
6.
7.
25
Summary of IUPAC system of
nomenclature
Find and name the longest continuous carbon chain.
Identify and name groups attached to this chain.
Number the chain consecutively, starting at the end
nearest a substituent group.
Designate the location of each substituent group by an
appropriate number and name.
Assemble the name, listing groups in alphabetical order.
The prefixes di, tri, tetra etc., used to designate several
groups of the same kind, are not considered when
alphabetizing.
Halogen substituents are easily accomodated, using the
names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-).
Examples of the IUPAC Rules in Practice
• By inspection, the longest chain is seen to consist of six carbons, so
the root name of this compound will be hexane .A single methyl
substituent (colored red) is present, so this compound is a
methylhexane. The location of the methyl group must be specified,
since there are two possible isomers of this kind. The IUPAC name
is thus 3-methylhexane.
26
Thus the parent chain will be the one with 4
substituents and the correct IUPAc name
of this compound is :
3-Ethyl-2,2,5-trimethylhexane
27
Important notes
•
•
The common names isopropyl, isobutyl, sec-butyl, tert-butyl are approved by the
IUPAC for the substituted groups.
Substituent groups are cited in the name in alphabetical order, regardless of their order
of occurrence in the molecule. Prefixes di, tri, tetra, and (tert written in italics and
separated from the name by a hyphen) are ignored, but prefixes iso, neo, and cyclo are
not!
Thus “tert-butyl” precedes “ethyl”, but ethyl preceeds “isopropyl”
7
9
5
6
10
1
4
3
2
6-tert-Butyl-2-methyl-decane
28
5
1
4
10
•
•
•
7
9
8
3-ethyl comes before 2,2-dimethyl
4-hexyl comes before 2,3-diisopropyl
3-Tert-butyl comes before 3-isopropyl
8
6
3
2
4-Isopropyl-3-methyl-decane
Physical Properties
• Methane, ethane, propane, and butane are gases;
pentane through hexadecane are liquids; the
homologues larger than hexadecane are solids.
• The boiling points and melting points of alkanes
increase with molecular weight.
• Branching reduces the boiling point, the more
branching the lower the boiling point.
•Alkanes are non- polar so are immiscible with
water , they are soluble in most organic solvents.
29
Preparation of alkanes
1- Hydrogenation of unsaturated hydrocarbon:
Ni or Pd or Pt / H2
H2C
CH2
H3C
CH3
200, 300
2- Hydrolysis of Grignard reagent
CH 3CH 2Br
+
2+
Mg
Dry ether
CH 3CH 2MgBr
Grignard reagent
CH 3CH 2MgBr
30
H3O
+
CH 3CH 3
+
Mg(OH)Br
3- Reduction of alkyl halides
a) by metal and acid or by metal hydrides
H
+ Zn
CH3CH2CH2Br
1) LiAlH4 / ether
CH3CH2CH2CH2Br
CH3CH2CH3
+ ZnBr2
CH3CH2CH2CH3
2) H3O
b) By sodium metal (Coupling reaction)
2 H3C
Br
+
2 Na
H3C
CH3
+
2 NaBr
c) By lithium dialkyl cuprate
(CH3CH2)2CuLi
31
+
CH3Br
CH3CH2CH3
Reactions of alkanes
• Chemically alkanes are very unreactive and stable at room
temperature towards acids , bases and most reactive
metals.
• Despite their relative inertness ( thus they known as
paraffines i.e lacking affinity) , alkanes undergo several
important reactions that are discussed in the following
section.
32
• 1- Halogenation
• Halogenation is the replacement of one or more hydrogen atoms in an
organic compound by a halogen (fluorine, chlorine, bromine or iodine).
•
The halogenation of an alkane appears to be a simple substitution reaction in
which a C-H bond is broken and a new C-X bond is formed; the reaction takes place
in presence of heat or UV light ( no reaction in the dark)
Heat
RH
RX
+ X2
or UV light
+ HX
Alkyl halide
X2 = Cl2 or Br2
• If there is one type of the carbon atoms in the molecule (e.g. methane
and ethane) H
H
C
H + Cl
Cl
excess
Heat or UV light
CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl
H
• If there are different types of carbon atoms in the molecule (Selectivity
issue)
When alkanes larger than ethane are halogenated, isomeric products are
formed. Thus chlorination of propane gives both 1-chloropropane and 233
chloropropane
as mono-chlorinated products.
H3C
°
1
CH3
2°
°
1
+ Br
Br
Heat or UV light
H3C
CH3
H3C
Br
+
Br
Major
Propane
Minor
2- combustion of alkanes (burning themdestroying the whole molecule )
CH4
CO 2 +
2 H 2O + 213 Kcal\mol
+ 2 O2
C2H6
+
7/2 O 2
2 CO 2
+
3 H 2O
+
373 Kcal\mol
160 kcal\mol for each methylene group (CH2)
34
POLLUTION
Processes involving combustion give rise to a variety of pollutants...
SO2 emissions produce acid rain
CO, NOx and unburnt hydrocarbons
power stations
internal combustion engines
react emitted gases with a suitable compound (e.g. CaO)
pass exhaust gases through a catalytic converter
Removal
SO2
CO and NOx
Catalytic converters
CO is converted to CO2 In the catalytic converter ...
NOx are converted to N2
Unburned hydrocarbons are converted to CO2 and H2O
e.g.
2NO + 2CO ———> N2 + 2CO2
• Catalysts are made of finely divided rare metals Rh, Pd, Pt
• Leaded petrol must not pass through the catalyst as the lead deposits on
the catalyst’s surface and “poisons” it, thus blocking sites for reactions to take
place.
35
Cycloalkanes
•Cycloalkanes are alkanes that have carbon atoms
forming rings (called alicyclic compounds)
•Simple cycloalkanes have the formula (CH2)n, or
CnH2n
•Nomenclature of Unsubstituetd Cycloalkanes
COMPOUNDS
1. Cycloalkanes with only one ring:
36
Naming Substituted Cycloalkanes
•Count the number of carbon atoms in the ring
and the number in the largest substituent chain.
If the number of carbon atoms in the ring is equal
to or greater than the number in the substituent,
the compound is named as an alkyl-substituted
cycloalkane.
•For an alkyl- or halo-substituted cycloalkane,
start at a point of attachment as C1 and number
the substituents on the ring so that the second
substituent has as low a number as possible.
•Number the substituents and write the name
37
with
the substituents in alphabetical order
37
38
39
• However if the alkyl sustituent is large and/or
complex, the ring may be named as a
substituent
group
on
an
alkane.
CH2CH2CH2CH2CH3
1
3
2
1-cyclobutylpentane
40
1,3-Dicyclohexylpropane
Cis-Trans Isomerism in Cycloalkanes
• Rotation about C-C bonds in cycloalkanes is limited
by the ring structure
• There are two different 1,2-dimethylcyclopropane
isomers, one with the two methyls on the same side
(cis) of the ring and one with the methyls on
opposite sides (trans)
41
41
Reactions of cycloalkanes
• Less stable rings
HI
CH 3CH 2CH 2I
H2SO4 / H2O
H2/Ni
Heat or UV
Br2/CCl 4
AlBr3
CH 3CH 2CH 2OH
CH 3CH 2CH 2CH 3
BrCH 2CH 2CH 2CH 2Br
• More stable 5 and 6 rings
CH3
CH3
Br2/UV or Heat
42
Cl 2/heat or UV
Br
Cl