21.1. INTRODUCTION
In chapters 21 and 22 we shall look at the reactions of different types of organic molecule. We shall attempt
to predict main reaction types from structure and then, for each type of molecule, we shall briefly
summarise reactions which do not easily fall into the 12 types described in the last chapter.
If you are unfamiliar with the types of molecule considered in these chapters, then chapter 27
(Nomenclature) should help. Look up each type of molecule as you consider its reactions.
Some indication of reaction conditions will be given. It is ridiculous to learn a whole list of conditions: if
they are needed for laboratory procedures they can be looked up.
However, reaction conditions also give an indication of the ease with which a reaction occurs. They should
certainly be absorbed at a sub-conscious level to help you acquire a feel for relative reactivities. Once you
have such a feel, you will be able to predict reaction conditions as accurately as can reasonably be expected.
If your examiners require more, they are wasting your time.
21.2. ALKANES
21.2.1. Predictions: Alkanes have no regions of either exposed nuclear charge or high electron density and
are therefore unaffected by either nucleophiles or electrophiles.
Moreover, there are no polarised bonds, so reactions occur homolytically, when they occur at all. In
addition, bonds are strong so reactive free radicals are needed to make alkanes react.
Finally, there are no multiple bonds, so addition is not possible. Nor is elimination favoured because this
would involve simultaneous attack on hydrogen atoms attached to two adjacent carbon atoms - an unlikely
event. The result of attack by free radicals on an alkane is therefore substitution i.e. the nett reaction is
homolytic substitution, via mechanism 1 (FIG. 20.1.).
21.2.2. Homolytic substitution in alkanes: examples of attacking free radicals:
21.2.3.
Other
reactions:
Two other
homolytic
reactions
undergone by
alkanes are
cracking and

combustion.
These are not
chain
reactions, but like homolytic substitution, the conditions needed for reaction are extreme, i.e. high
temperature:
i) cracking: The bonds break rather randomly in cracking reactions, producing a mixture of saturated and
unsaturated hydrocarbons.
................ .....400 - 700°C
2CH
3
CH
2
CH
3
(g).....g..... CH
4
(g) + CH
2
=CH
2
(g) + CH
3
CH=CH
2
(g) + H
2
(g)
.....................................C-C bond fission............ dehydrogenation
Table 21.1. Examples of free radical substitution in alkanes (see section 20.3 for
mechanism)

Radical Reagent Conditions Product(s)
lCl chlorine
gaseous and UV light
or in CCl
4
chloroalkane, dichloroalkanes etc
lBr bromine gaseous/heat/UV bromoalkane, dibromoalkanes etc
lSO
2
.OH fuming sulphuric acid heat
alkanesulphonic acid (salts =
detergents)
lNO
2
concentrated nitric
acid
heat/gas phase
nitroalkanes (mixture due to fission
etc.)
ii) combustion:
CH
4
(g).... +.... 2O
2
(g)....g.... CO
2
(g).... +.... 2H
2
O(g)
2C

2
H
6
(g).... +.... 7O
2
(g)....g.... 4CO
2
(g).... +.... 6H
2
O(g)
Note that combustion is an oxidation reaction. Alkanes may also undergo "autoxidation", by a free radical
chain mechanism. This can be initiated by light, or an "initiator". Typical initiators are substances which
produce free radicals, sometimes at higher temperatures or in the presence of light. Autoxidation is a bad
term because it implies that the process takes place in the absence of any other reactant. In fact, the
oxidising agent is atmospheric oxygen.
21.3. ALKENES
21.3.1. Predictions i) The double bond in alkenes is a region of high electron density which therefore
attracts electrophiles. Moreover, the molecule is unsaturated and the attack results in addition. i.e. nett
reaction is electrophilic addition (by mechanisms 8 and 9).
However, the implied connection between unsaturation and addition begs the question, "What favours
saturation over unsaturation?"
The answer is illustrated in the equation below. The electrons involved in the bonds resulting from addition
(bonds iii, iv and v), are held more tightly than they were before addition occured (bonds i and ii); electrons
are pulled away from the double bond and the Br-Br bond, into single bonds where they are held more
tightly.
The relative tightness with which the electrons are held before and after addition can be understood in the
following way. The pair of electrons in the p orbitals of double bond (i) are not particularly close to the two
carbon nuclei. They become more strongly held in the two s bonds (iii and v) since these are directly
inbetween the carbon and bromine nuclei.
Moreover, the two electrons in bond (ii) between two large bromine atoms, become more tightly held in

bonds (iii) and (v), where the smaller size of the carbon atoms makes the bonding electrons closer to the
nuclei.
The changes are reflected by (not explained by) the energy changes involved in the reaction.
21.3.2. Electrophilic addition to alkenes: examples of attacking electrophiles
Table 21.2. Examples of electrophilic addition to alkenes.
Electrophile reagent conditions main product(s)
d+.. d- *Bromine tetrachloromethane, r.t. -CHBr-CHBr-
*Br-Br
†Br-OH
†Bromine
in water,
room temp. Water
-CHBr-CHOH-
plus some-CHBr-CHBr-
The observable disappearance of
brown colour makes this reaction
useful as a test for double bonds.
**H-Hal H-Hal gaseous -CH
2
-CHHal-
H-OH
acid
catalysed
room temp., some alkenes
Industrially high temp and press
used. E.g. ethanol using silcon
dioxide coated w. H
3
PO
4

as
catalyst (see also method below)
-CH
2
-CHOH-
H-OSO
3
H
conc.
sulphuric
acid
room temperature
-CH
2
-CHOSO
3
H-
Boiling the product with water
gives alcohols by nucleophilic
substitution, an important
industrial process. 85% sulphuric
acid at 0°C is used for the
addition stage.
*Or chlorine (faster than bromine) or iodine (v.v.slow). (Fluorine reacts differently and
explosively with ethene to give carbon and HF gas.)
†Or chlorine, or iodine.
** reaction rate: HF << HCl < HBr < HI (Can you explain?)
21.3.3. Predictions ii) The above reactions occur in the absence of conditions which produce free radicals.
In conditions where free radicals are present (see section 20.12.1.) addition may occur via a homolytic
mechanism, i.e. homolytic addition via mechanism 6 (FIG. 20.1.).

Reactants which may add homolytically include: *Br-H (not HCl or HI), RS*-H, Cl
3
C*-Cl, and Cl
3
C*-H. *
indicates the part of the molecule which forms the initial attacking radical.
Polymerisation by a homolytic addition mechanism has already been discussed in section 20.12.2.
21.3.4. Another example of homolytic addition, but not a chain reaction, is the reduction of alkenes by
hydrogen gas in the presence of a metal catalyst such as platinum, or finely powdered nickel.
.......................................Pt/200°C
-CH=CH-...... +...... H
2
..........g.......... -CH
2
-CH
2
-
......................................fine Ni/r.t.
The binding sites on the nickel for hydrogen atoms are slightly further apart than the length of the H-H
bond. This tends to split the hydrogen into reactive atoms. Hydrogenation of double bonds is an important
process in the manufacture of some margarines. Saturated fats tend to be more solid than unsaturated oils,
though the health implications are well known.
21.3.5. Other reactions:
i) Oxidation:
a) Combustion:
CH
2
=CH
2
(g)...... +...... 3O

2
(g)..........g.......... 2CO
2
(g)...... +...... 2H
2
O(g)
b) With acidified potassium manganate (VII) solution:
................................................................cold dil.
-CH=CH-...... +...... H
2
O...... +...... [O]..........g.......... -CH-CH-
................................................................KMnO
4
....... |.... |
..................................................................................OH .OH
Note that this reaction involves a readily observable change. The purple colour of the manganate (VII)
disappears, and brown manganese (IV) oxide is precipitated.
c) With ozone:
............................................................O-O
.....\..... / .......................e.g. CCl
4
...... \ /.... \ /
......C=C...... +...... O
3
..........g........... C...... C
...../..... \........................ solution...... / .\ .../. \
..............................................................O
.......................................................an ozonide
Ozonides are explosive and are not isolated. However, hydrolysis of the ozonide is a useful reaction. It
produces carbonyl compounds (provided a reducing agent such as zinc dust and ethanoic acid is present to

prevent oxidation of the carbonyls by the hydrogen peroxide):
.........O-O............ H
2
O
.....\ /..... \ /...... r.t./warm...... \............................ /
......C...... C.............g............ C=O...... +...... O=C...... +...... H
2
O
2
...../. \... /. \ .......Zn/HEt........ /............................. \
..........O
The overall reaction with ozone, followed by hydrolysis, is known as ozonolysis and its usefulness lies in its
power as an analytical tool:
analysis of the resultant carbonyls gives information about the structure of the parent alkene. For example,
what alkene would produce a mixture of propanone and ethanal on ozonolysis?
21.4. ALKYNES
21.4.1. Predictions i) The arguments are similar to those for alkenes. The triple bond in alkynes is a region
of high electron density which therefore attracts electrophiles. Moreover, alkynes are unsaturated and
attack results in addition. The nett reaction is therefore electrophilic addition via mechanisms 8 and 9
(FIG. 20.1.).
Note that, as discussed in section 20.15, alkynes are often less reactive than alkenes to electrophiles, despite
their higher electron density.
Note also, that after addition to a triple bond, there is still a double bond. This may undergo further
electrophilic addition. However, reactivity may be less than expected if the first addition to the triple bond
has introduced, say, a halogen atom into the molecule:
A halogen atom attached to a doubly bonded carbon atom has a negative inductive effect (section 21.5.).
This reduces the electron density in the double bond and makes it less susceptible to electrophilic attack
than a double bond in a simple alkene. Moreover, further addition will be directed as predicted by
Markownikoff's rule (section 20.14.1.).
21.4.2. Examples of electrophilic addition to alkynes

The electrophiles which add to alkynes are largely the same as those which add to alkenes (table 21.2.), and
in the absence of free radicals, the main product is predicted by Markownikoff's rule. However, remember
that alkynes are generally less reactive than alkenes and:
(i) Bromine water does not react.
(ii) the addition of halogens or halogen halides requires a halogen carrier catalyst such as FeBr
3
.
Alternatively, UV light enables the reaction to proceed via a homolytic mechanism. However, under these
conditions, the reaction with chlorine may be explosive, producing carbon and hydrogen chloride.
(iii) addition of water under acid conditions requires mercury (II) sulphate to further catlyse the process.
The method used is bubbling the alkene into hot dilute sulphuric acid containing the catalyst. The "enol" so
produced is unstable and rapidly undergoes rearrangement to form a carbonyl compound. For example:
The reaction is useful in the synthesis of a large range of organic compounds, especially when it is
considered that carbon itself may be the starting point, via calcium(II) dicarbide!
...................................... 2000°C
CaO(s).... +.... 3C(s).............g............ CaC
2
(s).... +.... CO(g)
CaC
2
(s).... +.... 2H
2
O(l).............g............ Ca(OH)
2
(s).... +.... CH=CH(g)
21.4.3. Predictions ii) Apart from electrophilic addition there is another fascinating property of alkynes.
Electrons in an sp
1
orbital are closer to the nucleus than those in an sp
2

orbital, and even closer than those in
an sp
3
orbital. Under certain conditions, a hydrogen next to a triple bond can actually be removed as a
proton and the C-H bonding electron pair accomodated in the carbon atom's sp
1
orbital. Thus a carbanion is
formed and the alkyne can be regarded as having slight acidic properties (section 21.4.4.).
21.4.4. Acidic properties. The acidic properties are shown in two ways:
i) The amide ion is a strong enough base to remove the acidic hydrogen. The reagent is sodium dissolved in
liquid ammonia.
2NH
3
(l).... +.... 2Na(s).............g............ 2Na
+ -
:NH
2
(am).... +.... H
2
(g)
-C=C-H(g)......
-
NH
2
(am).............g............ -C=C
-
(am).... +.... NH
3
(g)
Sodium alkynides are extremely useful for the synthesis of other alkynes because the alkynide ion is a

powerful nucleophile in reaction with haloalkanes (section 21.7.2.).
(ii) Also, alkynes with terminal hydrogen atoms form silver and copper(I) salts when treated with diammine
complex ions of the metals. The formation of characteristic precipitates makes the reactions useful tests for
1-alkynes:
RC=CH(g) + Cu(NH
3
)
2
+
(aq) +
-
OH(aq)...g... RC=CCu(s) + H
2
O(l) + NH
3
(aq)
......................................................................red ppt.
RC=CH(g) + Ag(NH
3
)
2
+
(aq) +
-
OH(aq)...g... RC=CAg(s) + H
2
O(l) + NH
3
(aq)
....................................................................white ppt.

21.4.5. Other reactions
i) Oxidation: Like alkanes and alkenes, alkynes undergo various oxidation reactions, not least autoxidation
and combustion.
E.g. Combustion: 2CH=CH(g)... +... 5O
2
(g)...g... 4CO
2
(g)... +... 2H
2
O(g)
21.5. INDUCTIVE AND MESOMERIC EFFECTS
21.5.1. Introduction: In section 21.4.1. a new concept was slipped into the text without explanation. What
is a negative inductive effect? For that matter, what is a positive inductive effect? Briefly, inductive effects,
positive or negative, are little more than polarised bonds seen with a different journalistic bent. It is
important to realise that even scientific language depends on the attitude of the observer.
Inductive effects exist in s-bonds and also in p-bonds, but in the former case they do not involve
delocalisation. Polarisations which do involve delocalisation via p bonding systems and p-orbitals are
known as mesomeric or conjugative effects.
In fact, mesomeric and conjugative effects are little more than delocalisation seen with a different
journalistic bent. They do not even involve polarisation in all circumstances.
Two further points on language: First, the different jounalistic bent described above is not totally artificial.
It is useful for describing particular situations because it saves clumsy explanations. Good scientists would
not make good Sun reporters, though they might do well on The Independent.
Second, inductive and mesomeric effects are often talked about as "occurring". This does not mean that they
occur on any time scale. The negative inductive effect of a halogen atom does not suddenly happen in a
haloalkane; it is there all the time.
21.5.2. Inductive effects exist where (occur where) two atoms or groups which differ in electronegativity
are bonded.
A more electronegative atom or group exerts a negative inductive (-I) effect, "pulling" electrons towards
itself and acquiring partial negative charge (d-).

For example, chlorine and oxygen exert -I effects in the molecules shown here:
The most important groups to exert an electron pushing, or +I, effect are alkyl groups. This is largely a
characteristic of the large number electropositive hydrogen atoms within alkyl groups.
21.5.3. Mesomeric or conjugative effects exist:
i) where p-bonding systems would otherwise be next to each other - separated by one single bond, or
ii) where electrons in p-orbitals would otherwise be next to p-bonding systems - separated by one single
bond.
The term conjugation is often reserved for situations where the polarity of the effect is not relevant, eg in
buta-1,3-diene (section 4.8.8.) The double bonds which appear in the simple bonding diagram (FIG. 4.13.)
are conjugated and there is no polarisation. However, in phenylethene, it is more relevant to think of the
ethene group exerting a positive mesomeric (+M) effect on the benzene ring. The p-bonding systems are
conjugated, but in this case there is polarisation:
Another way of describing the situation is to say that electrons from the alkene double bond are delocalised
into the benzene ring.
In phenylethanal, the carbonyl group is considered as exerting a negative mesomeric (-M) effect on the
benzene ring. Electrons are delocalised out of the ring onto the electronegative oxygen atom:
Thus doubly bonded electronegative elements exert negative mesomeric effects as well as negative
inductive effects. In other cases, the polarity of mesomeric effects is determined by the relative electron