Download Cyclopropane CH 2 CH 2 CH 2 Cyclobutane CH 2 CH 2

Chemical Ideas 12
Organic chemistry
frameworks
12.1 Alkanes

Many carbon compounds are found in living
organisms, this why their study is named
organic chemistry
Carbon is unique
About 7 million compounds
containing carbon and hydrogen
are known to chemists
This is far more than the
number of compounds from all
the other elements put together
Why carbon?



Electron structure makes it the first member of
Gp 4 in the centre of the periodic table – this is
responsible for its special properties
Carbon atom has 4 electrons in its outer shell – too
many to lose or gain – the ions would have +4 or -4
(too highly charged)
All carbon compounds are covalent rather than
ionic

Methane(CH4) C shares 4 electrons with 4 H atoms

Carbon forms strong covalent bonds with itself to
form rings and chains this is called catenation

Each C atom can form 4 covalent bonds- chains
may be straight or branched and can have other
atoms or groups substituted on to them
H
 
H  C  H
 
H
Hydrocarbons


Only contain carbon and
hydrogen
General formula CXHY
Methane –CH4 is an alkane
Ethene C2H4 is an alkene
Benzene C6H6 is an arene
H
l
H - C - H
l
H
Alkanes
 Saturated hydrocarbons
 Contain C and H only
 Contain single bonds C-C
 Have 4 bonds to every carbon (C) atom
 Are non polar
 General formula CnH2n+2
 Physical properties such as m.pt,b.pt and density change
as the number of carbon atoms in the molecule increase
Alkanes
Name
Names of all alkanes end in -ane
# carbons
shortened
Structural Formula
Methane
1
CH4
Ethane
2
CH3CH3
Propane
3
CH3CH2CH3
Butane
4
CH3CH2CH2CH3
Pentane
5
CH3CH2CH2CH2CH3
Alkanes
Names of all alkanes end in -ane
shortened
Name
# carbons
Structural Formula
Hexane
6
CH3CH2CH2CH2CH2CH3
Heptane
7
CH3CH2CH2CH2CH2CH2CH3
Octane
8
CH3CH2CH2CH2CH2CH2CH2CH3
Nonane
9
CH3 CH2 CH2CH2CH2CH2CH2CH2CH3
Decane
10
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
A series of compounds which are related in this way
are called a homologous series
Finding the formula of alkanes
Molecular formula = m x empirical formula
Where m is 1,2,3,……

For hydrocarbons , composition of mass is
easily found by burning a known mass in
oxygen, measuring the amounts of CO2 and
H2O this is called combustion analysis
Finding the formula of alkanes
Example
0.100g of hydrocarbon X on complete combustion gave
0.309g CO2 and H2O 0.142g
calculate the empirical formula of X
Answer
1st calc the masses of C and H in 0.100g
44g CO2 contains 12g of C
mass of C in 0.100g X = (12/44) x 0.309g
= 0.0843g
18g H2O contains 2g H
mass of H in 0.100g X = (2/18) x 0.142g
= 0.0158g
Finding the formula of alkanes
C
0.0843
0.00703
Ratio by mass
Ratio by moles
Simplest by
ratio(÷ by smaller)
Whole number
ratio
Empirical formula = C4H9
:
:
H
0.0158
0.0158
1
:
2.25
4
:
9
Finding the formula of alkanes
Example
Relative molecular mass of X was found to
be 114 by using a mass spectrometer
Answer
Empirical formula of X is C4H9
but Mr C4H9 = 57 this is ½ of 114. So the
molecular formula of X must be (C4H9)2
= C8H18
Structure of alkanes




Full structural formula of
methane
Shows all atoms and bonds
Propane
Shortened structural formula
for propane
Further shortened to
Structure of alkanes
Alkyl Groups
Branches on carbon chains
H
H
H
C
CH3
H
H
H
C
C
H
H
CH3CH2
methyl
ethyl
Naming organics –
nomenclature
Ref. P136 Organic Chemistry Solomons and Fryhle 2002
International Union of Pure and Applied
Chemistry – IUPAC – has an internationally
agreed and accepted method for giving evey
organic molecule a unique name using a
method - you are going to be examined on
your ability to use this methodology. It is
worth spending time learning and practicing
this skill. Very useful indeed. The method is
fairly easy and stepwise as follows:
Naming organics –
nomenclature
Ref. P136 Organic Chemistry Solomons and Fryhle 2002
1. Locate the longest continuous chain of
carbon atoms; this chain determines the
parent name for the alkane – this may not
always be obvious since they go around
corners!
Naming organics nomenclature
2. Number the longest chain beginning with the
end of the chain nearer the substituent
Naming organics nomenclature
3. Use the numbers obtained from rule 2 to
designate the location of the substituent
group
Naming organics nomenclature
4. When there are more than one group on
different parts of the chain – number from the
longest chain for example we call this
“4-ethyl-2-methylhexane”
Naming organics nomenclature
5. When two substituents are on the same
carbon atom, use that number twice for
example, this is 3-ethyl-3methylhexane
Naming organics nomenclature
6. When two or more substituents are identical,
indicate this by the use of the prefixes di-, tri-,
tetra-, and so on. Make sure every substituent
has a number. Commas separate numbers from
each other. 2,3-dimethylbutane
Naming organics nomenclature
7. Classification of hydrogen atoms
2-methyl butane has primary (1o), secondary
(2o) and tertiary (3o) hydrogens
Naming organics nomenclature
8. If there is a halide group – p140 – number
from the first substituent attached to it
regardless of whether it is halo or alkyl. If
they are equal distance, then go
alphabetically.
Naming alkenes nomenclature
1. For alkenes, determine the parent by
selecting the longest chain that contains the
double bond and change the ending of the
name from “–ane” to “-ene”
Naming alkenes nomenclature
2. Number the chain so as to include both
carbon atoms of the double bond and begin
naming numbering at the end of the chain
nearest the location of the double bond
Naming alkenes nomenclature
3. Number the other substituents as we have
already learned.
Naming alcohols –
nomenclature (p141)
In what is called IUPAC substitutive
nomenclature a name may have as many as
4 features! Locant, prefix, parent compound
and one suffix….(continuted)
Naming alcohols –
nomenclature
…For the alcohols, you add “-ol” to the suffix, in
general, numbering of the chain always
begins at the end nearer the group named as
a suffix.
Naming alcohols –
nomenclature (p141)
1. Select the longest chain again to which the
hydroxyl (OH) group is attached. Change
the name of the alkane corresponding to this
chain by dropping the “e” and adding the
suffix “ol”
Naming alcohols –
nomenclature (p141)
2. Number the longest continuous chain so as
to give the carbon atom bearing the hydroxyl
group the lower number.
Naming Summary
1. Count the C’s in the longest chain
2. Name each attached group
3 Count the longest carbon chain to
give the first attached group the
smallest number
4. Name and locate each group
Naming Branched Alkanes
CH3
methyl branch
CH3CH2CH2CHCH2CH3
6
5
4
3
2
1
Count
3-Methylhexane
on third C
CH3
six carbon chain
group
Branched alkanes -Isomers

Same molecular formula

Same number and types of atoms

Different arrangement of atoms
Cycloalkanes
•The is a group of alkanes that have
a cyclic structure.
•These cycloalkanes contain a
carbon chain that is in a ring.
•Each cycloalkane has a formula that
is 2C less than the corresponding
alkane.
•For example, propane is C3H8
whereas cyclopropane ic C3H6.
Butane is C4H10 and cyclobutane is
C4H10.
•The names of the cyclic structures
use the prefix cyclo in from of the
alkane name for the carbon chain
Cyclopropane
CH2
CH2
CH2
Cyclobutane
CH2
CH2
CH2
CH2
More Cycloalkanes
Cyclopentane
CH2
CH2
CH2
CH2
CH2
Cyclohexane
CH2
CH2 CH2
CH2
CH2
CH2
Shapes of alkanes
Pairs of electrons in
the bonds repel
each other so in all
covalent compounds
the bonds are as far
away from each
other as possible.
The bond angle for
H-C-H is 109
represents a bond in
the plane of the paper
represents a bond in
a direction behind
the plane of the paper
represents a bond in
a direction in front of
the plane of the paper
Shapes of alkanes
Ethane
Each carbon atom is at the centre at
of a tetrahedral arrangement
a simpler way of drawing
ethane which shows the
shape less accurately
Shapes of alkanes
Hydrocarbon chains are not really straight but a zig-zag of carbon
atoms. All bond angles are 109◦
Reaction of alkanes


Oxidation are v. unreactive unaffected by
acids and alkalis and oxidising agents
When they do react it is usually in the gas
phase and energy needs to be supplied to
get the reaction started
Reaction of alkanes
Combustion
alkane + O2

CO2 + H2O + heat
C6H14 + 9 1/2 O2  6CO2 + 7H2O + heat

If air supply is limited combustion in incomplete
and products include CO and C (soot) along with
partially oxidised hydrocarbons

Action of heat on alkanes
When alkanes fractions are heated under different conditions,3 different
reactions can occur, isomerisation, reforming and cracking