1. Unit 2.8 - Organic chemistry - alcohols and halogenoalkanes
Revision - IUPAC system for naming organic compounds
You should remember from unit 1 that there is a set of rules, set by IUPAC, for classifying and
naming organic compounds. As we will be studying further organic chemistry in this unit now is a
good time to remind yourself of those rules:
Number of
Position of functional groups in the chain
Prefix Carbons in
chain
1. The 'parent' chain is the ___________ possible chain
meth-
2. The carbon with the functional group attached is given
eth-
a ____________.
prop-
3. Numbering starts from the end of the chain that gives
the carbon being considered the ____________ number.
but-
pent-
Alkyl groups
hex-
methyl
hept-
1. Position in chain is ethyl
indicated by
oct-
___________ possible propyl
number.
dec-
2. If more than 1 side chain is attached then the name
dodec-
of the compound includes the side groups in
_________________ order, regardless of which
eicos- carbon atom they are found on.
Try these examples:
4-methylpent-1-ene E-hex-2-ene
1. Alcohols
Introduction
Ethanol is the compound generally referred to as alcohol and found in
alcoholic drinks. It is most commonly produced through the action of yeast
2. on natural sugars otherwise known as fermentation. Many other alcohols do exist; in chemistry
‘alcohol’ refers to the homologous series of molecules containing the hydroxyl (- OH) functional
group. The reactivity of alcohols is based on the hydroxyl group. They are industrially important as
raw materials in the synthesis of other compounds and as solvents.
Naming
Naming alcohols is relatively simple - the name of the alkyl group containing the ______________
number of carbons is used and the suffix ______ is added. Here are some examples:
Name Displayed formula Skeletal formula
Methanol
Ethanol
Propan-1-ol
________pentan-2-
ol
Primary, secondary or tertiary?
There are three main types of alcohol - primary , secondary and tertiary ; they are defined by the
number of alkyl (- C) groups attached to the carbon with the hydroxyl (-OH) group:
Primary alcohols
Ethanol
Primary alcohols have the general structure RCH2OH. There are two hydrogens and one alkyl
group attached to the same carbon as the hydroxyl group
3. Secondary alcohols
Propan-2-ol
Secondary alcohols have the general structure RR1CHOH. There is one hydrogen and two
__________ groups attached to the same carbon as the hydroxyl group
Tertiary alcohols
2-methylpropan-2-ol
Tertiary alcohols…
NB These differences in structure are very important as they affect the way that the alcohol
reacts!
Reactions of Alcohols - combustion, reduction, oxidation and substitution
a. Combustion
Alcohols burn readily in air or even more readily in pure oxygen. Many burn with an almost
unperceivable blue flame. Combustion of alcohols produces carbon dioxide and _________
CH3CH2OH(l) + ___O2(g) → ___CO2(g) + ___H2O(l)
CH3CH2CH2CH2OH(l) + ___O2(g) → ___CO2(g) + ___H2O(l)
b. Reaction with sodium
• Sodium reacts vigorously with water to produce _________________ and ____________.
The sodium is less dense than water so it floats, fizzes and moves around. Water acts as
an acid in this reaction by losing a proton
• The reaction with alcohols is _________vigorous and the sodium _________ in the ethanol
as it is _________ dense than the alcohol
2Na(s) + 2CH3CH2OH(l) → 2CH3CH2O-Na+(alc) + H2(g)
sodium ethoxide
• The products are _________________ and ___________, the reaction is less vigorous
because ethanol is a _____________ acid than water
4. c. Oxidation
Oxidation reactions are a useful test to find out if an alcohol is primary, secondary or tertiary
as they each give different products. Common oxidising agents such as acidified
potassium dichromate(VI) and potassium manganate(VII) can be used…
• Primary alcohols - Are readily oxidised to form aldehydes, which then rapidly oxidise to
form carboxylic acids:
+ H2O
Propan-1-ol Propanal Propanoic acid
Primary alcohol Aldehyde Carboxylic acid*
*The most common product of this oxidation is the carboxylic acid; however, the aldehyde can
be separated during the reaction
• Secondary alcohols - Are readily oxidised to form ketones with no further oxidation
occurring:
Propan-2-ol Propanone
Secondary alcohol Ketone
• Tertiary alcohols - Are not oxidised by any of the common oxidising reactions, there is
simply no reaction
Oxidation as a test for primary, secondary or
tertiary
Add acidified potassium dichromate (VI)
Primary -
Secondary -
Tertiary -
Chromium(III) compounds are dark green
5. Distillation - separating the products of oxidation
• Distillation can be used to prepare a ketone
from a secondary alcohol:
Compound Boiling temp (°C)
Butanone 80
Butan-2-ol 95
Only the ketone will be collected if the
temperature is kept between 80 °C & 94
°C
• Or to separate the products of
oxidation of a
primary alcohol:
Compound Boiling temp
(°C)
Ethanol 78
Ethanal 21
Ethanoic acid 118
The aldehyde can be collected between
21 °C & 70 °C
Reflux - preparing carboxylic acids from primary alcohols
Heating under reflux ensures that the oxidation reaction is completed
Any aldehyde or unreacted primary alcohol
evaporates, condenses here in the
_______________ and falls back into the
reaction vessel where it can it oxidised
The oxidation reaction takes place here. First the aldehyde
is produced then, after further oxidation, the
________________. Heat causes the products to
_____________.
d. Substitution to form halogenoalkanes
Alcohols react with phosphorous(V) chloride (PCl5) to produce a chloroalkane and hydrogen chloride
gas…
CH3CH2OH(l) + PCl5(s) → CH3CH2Cl(l) + HCl(g)
NB this reaction can be used as a test for alcohols - if you add PCl5 to an unknown liquid the
production of HCl gas is evidence for the presence of an -OH group.
Preparation - halogenoalkanes from alcohols
e.g. the preparation of 1-bromobutane from butan-1-ol…
6. Step 1 - Sodium bromide reacts with conc. sulphuric to produce hydrogen bromide
NaBr(s) + H2SO4(l) → NaHSO4 + HBr(g)
Step 2 - Hydrogen bromide reacts with alcohol to produce 1-bromobutane
CH3CH2CH2CH2OH(l) + HBr(g) → CH3CH2CH2CH2Br(l) + H2O9(l)
Step 3 - The products are distilled to yield a mixture, which separates into 2 layers -
aqueous
and organic, a separating funnel is used to collect the organic layer
Step 4 - The aqueous layer is discarded and the organic layer is redistilled to produce 1-
bromobutane, which has a boiling temperature of 102 °C
2. Halogenoalkanes
Introduction
Halogenoalkanes are saturated hydrocarbons with one or several halogen functional groups. The
halogens are found in group ___ and consist of __________, ___________, __________ and
____________.
Uses of halogenoalkanes (p214-
p217)
Until 1986 chlorofluorocarbons (CFCs) were routinely used as
refrigerants in fridges and freezer and propellants in aerosols;
however, it was discovered that they undergo radical reactions
when exposed to UV radiation high in the atmosphere, which lead
to the removal of ozone (O 3 ) from the ozone layer…
CCl2F2 → •CClF2 + Cl•
Cl• + O3 → ClO• + O2
ClO• + O3 → Cl• + 2O2 Net reaction: 2O 3 → 3O 2
As the ozone layer protects us from harmful UV radiation, the international community chose to ban
the use if CFCs. Unfortunately many CFCs are very unreactive and will persist in the atmosphere for
hundreds of years.
Naming
Naming halogenoalkanes is relatively simple and follows the IUPAC convention…
• Prefix with fluoro, chloro, ____________ or _____________ depending on the halogen
• More than one halogen group can be indicated by adding di, tri or tetra
• Numbers are used to indicate the position of the halogen group in the chain
7. _________________ __________________ __________________ __________________
Primary, secondary or tertiary?
Just like alcohols, halogenoalkanes can be either primary, secondary or tertiary depending on the
number of alkyl groups attached to the carbon with the halogen group…
Primary halogenoalkanes
Chloroethane
Primary halogenoalkanes have the general structure RCH2X. There are two hydrogens and one
alkyl group attached to the same carbon as the halogen group
Secondary halogenoalkanes
2-chloropropane
Secondary halogenoalkanes have the general structure RR1CHX. There is one hydrogen and two
__________ groups attached to the same carbon as the halogen group
Tertiary halogenoalkanes
2-bromo-2-methylpropane
Tertiary halogenoalkanes…
Primary, secondary and tertiary halogenoalkanes react very differently. Luckily there is a test, which
8. can be used to identify which is present using silver nitrate solution. We will cover the test in the
reactions section under hydrolysis.
Reactions - hydrolysis, elimination and substitution
As mentioned on the previous page, the position of the halogen in the hydrocarbon chain affects
reactivity, as does the type of halogen present…
Bond Average bond energy
(kJmol -1 )
C-X bonds become ______________ down the C-F 467
C-Cl 346
group, meaning that fluorocarbons are very C-Br 290
_______________ and iodocarbons are C-I 228
relatively ______________
a. Hydrolysis - ‘splitting with water’
Halogenoalkanes react with water or hydroxide (OH - ) ions in a reaction known as hydrolysis. The
products are an alcohol and a hydrogen halide : RX + H2O → ROH + HX
RX + OH- → ROH + X-
e.g. CH3Cl + H2O → _____ + _____
CH3CH2CH2Br + H2O → _____________ + _____
NB Reactivity depends on the H-X bond strength as shown in the table above. Fluorocarbons are the
least likely to undergo hydrolysis
Test for primary, secondary or tertiary - silver nitrate
Procedure
• Add 1 cm3 of silver nitrate and 1
cm3 ethanol (__________) to each
tube and warm
• Add 5 drops of the different
halogenoalkanes and observe
Explanation
• Halogenoalkanes react with water
to form an _____________ and
1-bromobutane 2-bromobutane 2-bromo-2-
some _____________ ions
methylbutane
• Silver nitrate (AgNO3) reacts with
Primary
the halide ions to form an insoluble
_______________
• Rate of reaction:
_________ > _________ > _________
9. b. Reaction with potassium hydroxide
Potassium hydroxide (KOH) can react with halogenoalkanes in one of two ways depending on
whether it is dissolved in water or ethanol :
Conditions Type of reaction Product
i) Reflux heating with aqueous
Substitution Alcohol
KOH (dissolved in _________)
ii) Reflux heating with
alcoholic KOH (dissolved in Elimination Alkene
__________)
For example:
i) CH3CHBrCH3(l) + KOH(aq) → ____________ + ______
2-bromopropane aqueous KOH
ii) CH3CHBrCH3(l) + KOH(alc) → ____________ + ______
2-bromopropane alcoholic KOH
c. Reaction with alcoholic ammonia
There is a ________ pair of electrons on the ____________ atom of
ammonia (NH3), this means it can act as a nucleophile and replace
the halogen atom in a halogenoalkane:
i) CH3CH2I + NH3 → CH3CH2NH2 + HI
ethylamine
Ethylamine also has a lone pair of electrons on nitrogen so it can react further with iodoethane and
there is a chain reaction*:
ii) CH3CH2I + CH3CH2NH2 → (CH3CH2)2NH + HI
iii) CH3CH2I + (CH3CH2)2NH → (CH3CH2)3N +HI
iv) CH3CH2I + (CH3CH2)3N → (CH 3 CH 2 ) 4 N + I -
tetraethylammonium iodide
*Skeletal formulae for the amine products reactions i-iv respectively:
10. Spider diagram of A-level organic chemistry reactions
Task - Produce your own spider diagrams to represent the reactions of alcohols and
halogenoalkanes that we have covered. Be sure to include:
• Reactants
• Conditions
• Products