Transcript 9. Introducing Organic Chemistry
1 of 30 © Boardworks Ltd 2009
2 of 30 © Boardworks Ltd 2009
Molecular and empirical formulae
There are many ways of representing organic compounds by using different formulae.
The
molecular formula
of a compound shows the number of each type of atom present in one molecule of the compound.
The
empirical formula
of a compound shows the simplest ratio of the atoms present.
Molecular formula
C 2 H 6 C 6 H 12 O 6 C 2 H 4 O 2
Empirical formula
CH 3 CH 2 O CH 2 O Neither the molecular nor empirical formula gives information about the structure of a molecule.
© Boardworks Ltd 2009 3 of 30
Displayed formula of organic compounds
The
displayed formula
of a compound shows the arrangement of atoms in a molecule, as well as all the bonds.
Single bonds are represented by a single line, double bonds with two lines and triple bonds by three lines .
The displayed formula can show the different structures of compounds with the same molecular formulae.
4 of 30
ethanol (C 2 H 6 O) methoxymethane (C 2 H 6 O)
© Boardworks Ltd 2009
Structural formula of organic compounds
The
structural formula
of a compound shows how the atoms are arranged in a molecule and, in particular, shows which functional groups are present.
Unlike displayed formulae, structural formulae do not show single bonds, although double/triple bonds may be shown.
CH 3 CHClCH 3 H 2 C=CH 2 CH 3 C ≡N 2-chloropropane
5 of 30
ethene ethanenitrile
© Boardworks Ltd 2009
Displayed and structural formula activity
6 of 30 © Boardworks Ltd 2009
Skeletal formula of organic compounds
The
skeletal formula
of a compound shows the bonds between carbon atoms, but not the atoms themselves. Hydrogen atoms are also omitted, but other atoms are shown.
7 of 30 © Boardworks Ltd 2009
Types of formulae
8 of 30 © Boardworks Ltd 2009
Functional groups and homologous series
A
functional group
is an atom or group of atoms responsible for the typical chemical reactions of a molecule.
A
homologous series
is a group of molecules with the same functional group but a different number of –CH 2 groups.
methanoic acid (HCOOH) ethanoic acid (CH 3 COOH) propanoic acid (CH 3 CH 2 COOH)
Functional groups determine the pattern of reactivity of a homologous series, whereas the carbon chain length determines physical properties such as melting/boiling points.
9 of 30 © Boardworks Ltd 2009
Functional groups
10 of 30 © Boardworks Ltd 2009
Homologous series and general formulae
The
general formula
of a homologous series can be used to calculate the molecular formula of any member of the series by substituting
n
for the number of carbon atoms.
For example, the general formula of a halogenoalkane is
C
n
H 2n+1 X
, where X is a halogen.
Example: what is the molecular formula of chloroethane? 1.
Write down the general formula:
2.
Write down the value of
n
:
C
n
H 2n+1 n = 2 X 3.
Substitute
n
into the general formula:
C 2 H 5 Cl
© Boardworks Ltd 2009 11 of 30
Homologous series
12 of 30 © Boardworks Ltd 2009
13 of 30 © Boardworks Ltd 2009
What is isomerism?
Isomers
are molecules with the same molecular formula (i.e. the same number and type of atoms) but in which the atoms are arranged in a different way.
There are two main categories of isomerism:
structural isomerism
and
stereoisomerism
.
Structural isomers have different structural formulae.
Three types of structural isomerism are
chain isomerism
,
positional isomerism
and
functional group isomerism
.
Stereoisomers have the same structural formula, but the 3D arrangement of atoms is different. Two types are
cis
–trans isomerism
and
optical isomerism
. © Boardworks Ltd 2009 14 of 30
Chain isomerism in alkanes
In
chain isomers
, the carbon chain is arranged differently. For example, hexane has several chain isomers, all with the molecular formula C 6 H 14 :
hexane
15 of 30
3-methylpentane 2,3-dimethylbutane
© Boardworks Ltd 2009
Positional isomerism
In
positional isomers
, the functional group is attached to a different carbon atom.
For example, chloropentane has several positional isomers, all with the molecular formula C 5 H 11 Cl:
1-chloropentane 2-chloropentane 3-chloropentane
© Boardworks Ltd 2009 16 of 30
Positional isomerism in alkenes
Positional isomerism also exists in alkenes with four or more carbon atoms.
hex-1-ene
For example, hexene has several positional isomers, all with the molecular formula C 6 H 12 :
hex-2-ene hex-3-ene
© Boardworks Ltd 2009 17 of 30
Functional group isomerism
Functional group isomers
contain different functional groups and so are members of different homologous series. For example, both alcohols and ethers have the general formula C
n
H 2
n
+2 O so they may be functional group isomers:
propanol (C 3 H 8 O) an alcohol
18 of 30
methoxyethane (C 3 H 8 O) an ether
© Boardworks Ltd 2009
Structural isomers activity
19 of 30 © Boardworks Ltd 2009
20 of 30 © Boardworks Ltd 2009
Rotation around the C=C bond in alkenes
Molecules can rotate freely around single C-C covalent bonds, but not around C=C double bonds. This leads to type of stereoisomerism called
cis
–trans isomerism
, in which isomers differ in the arrangement of the groups attached to the carbons in the double bonds.
is not the same as
These isomers cannot be superimposed on each other because the arrangement of the methyl groups is different.
21 of 30 © Boardworks Ltd 2009
Cis
–trans isomerism
If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named either
cis
(‘on the same side’) or
trans
(‘on the opposite side’).
cis-but-2-ene trans-but-2-ene cis-1,2-dichloroethene
22 of 30
trans-1,2-dichloroethene
© Boardworks Ltd 2009
Limitations of cis –trans isomerism
In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the
cis
–
trans
notation.
For example, is it possible to identify which of these halogenoalkanes is the
cis
isomer and which is the
trans
isomer?
Instead, a different system is used for these type of molecules:
E –Z notation
.
23 of 30 © Boardworks Ltd 2009
E –Z isomerism
The
E –Z notation
is used to identify stereoisomers that cannot be called
cis
or
trans
.
Isomers are identified as either E or Z depending on what ‘priority’ is given to the groups attached to the carbon atoms in the double bond. The priority of these groups is determined by a complex series of rules.
E
represents the German word ‘
entgegen
’, and corresponds to
trans
isomers. The highest priority groups are on the
opposite
side of the double bond.
Z
represents the German word ‘
zusammen
’, and corresponds to
cis
isomers. The highest priority groups are on the
same
side of the double bond.
© Boardworks Ltd 2009 24 of 30
Optical isomerism
Another form of stereoisomerism is
optical isomerism
, in which a molecule can exist as two isomers that are non superimposable, mirror images of each other, just like a left hand and right hand.
optical isomers of the amino acid alanine
Optical isomers have the same physical properties, but they rotate
polarized light
in opposite directions.
25 of 30 © Boardworks Ltd 2009
Stereoisomerism: true or false?
26 of 30 © Boardworks Ltd 2009
27 of 30 © Boardworks Ltd 2009
Glossary
28 of 30 © Boardworks Ltd 2009
What’s the keyword?
29 of 30 © Boardworks Ltd 2009
Multiple-choice quiz
30 of 30 © Boardworks Ltd 2009