Download cape chemistry unit ii module i

CAPE CHEMISTRY UNIT II MODULE I
Alcohols and phenol and Alkenes Worksheet and Revision guide
1.
First part of homework - Due October 3, 2008
(a)
Describe the reaction of butan-2-ol with iodine in sodium hydroxide.
(b)
Describe the reaction between phenol and sodium hydroxide
2.
Classwork (websites for additional notes included with each section)
(a) The structure of alcohols
 Alkyl group- + -OH
 Oxygen more electronegative than carbon and hydrogen
 C is +, H is + and O is  Note – bent structure around O which have 2 lone pairs
(b) Classes of alcohol
http://www.chemguide.co.uk/organicprops/alcohols/background.html#top
The carbon bearing the –OH group:
 Primary - has one R group and two hydrogens
 Secondary - has two R groups and one hydrogens
 Tertiary - has three R groups
(c)
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Two types reaction of alcohols
Fission of the RO-H bond
Fission of the R-OH bond
Alcohols has tendency for H+ to dissociate in presence of a base
Alcohols act like acids [weaker acid than water]
(d) Fission of RO-H bond
http://www.chemguide.co.uk/organicprops/alcohols/sodium.html#top
(i)
reaction with sodium  H2(g) and Na alkoxide
ethanol + Na  H2(g) + Na ethoxide
propanol + Na  H2(g) + Na propoxide
(ii)
reaction with carboxylic acid (esterifcation)
http://www.chemguide.co.uk/organicprops/alcohols/esterification.html#top
propanol and butanoic acid  propyl butanoate + water
butanol and propanoic acid  butyl propanoate + water
Catalyst – HCl or conc. H2SO4
(e) Fission of the R-OH bond
(i) Halogenation using: HCl, HBr, HI, PBr3, PCl5
[mention only]
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 1 OF 9
(ii) Dehydration – reaction with conc. H2SO4 (as catalyst)
http://www.chemguide.co.uk/organicprops/alcohols/dehydration.html#top
A primary alcohol reacts with cold concentrated sulphuric acid to form alkyl hydrogensulphate :
Ethanol + conc. sulphuric acid  Ethyl hydrogensulphate + Water
If the alcohol is in excess and the reaction mixture is warmed to 140oC, an ether is formed
Ethyl hydrogensulphate + ethanol (excess)  Diethyl ether (ethoxyethane) +sulphuric acid
If the concentrated sulphuric is in excess and the temperature is raised to 170oC, water is eliminated,
with the formation of an alkene:
ethyl hydrogensulphate + conc. sulphuric acid (excess)  ethene + sulphuric acid
Dehydration – mainly 3o alcohol – carbocation stabilized by alkyl groups
(iii) oxidation
 Combustion of 1o, 2o and 3o– degradation of C skeleton  CO2, H2O
 Reaction with oxidants - maintain C skeleton
Reactions with: KMnO4 / H+; K2Cr2O7 / H+ Or Na2Cr2O7/ H+
http://www.chemguide.co.uk/organicprops/alcohols/oxidation.html#top

KMnO4/H+ is a stronger oxidizing agent than K2Cr2O7

Oxidation product depends on the class of alcohol
 With K2Cr2O7 / H+ or Na2Cr2O7 / H+
1 – oxidized to aldehydes and further oxidation to carboxylic acid if temperature is raised or if aldehyde
is not distilled off
2o - oxidized to ketones
3o – resistant to oxidation
o
 With KMnO4/H+
1 – oxidized directly to carboxylic acid
2o - oxidized to ketones
o
Primary alcohol
Primary alcohol: -OH on C with 2 hydrogens attached [-OH on C attached to 1 other C]
During oxidation: [-2H] one from –OH and one other
Result – aldehyde
Aldehyde: 1 H attached to the carbonyl C
Further oxidation to the carboxylic acid
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 2 OF 9
To obtain the aldehyde – distill off as it is formed
Aldehyde has lower b.p. than alcohol – no hydrogen bonds.
Primary alcohol  aldehyde  carboxylic acid
e.g. propan-1-ol, propanaldehyde, propanoic acid
Secondary alcohol
With KMnO4/H+, K2Cr2O7 / H+ or Na2Cr2O7 / H+
Secondary alcohol: -OH on C with 1 hydrogen attached
[-OH on C attached to 2 other C]
During oxidation: [-2H] one from –OH and one other
Result – ketone
The carbonyl carbon in a ketone does not have a hydrogen- no further oxidation (ketones resist oxidation)
Tertiary alcohol
Tertiary alcohol: -OH on C with 0 hydrogen attached
[-OH on C attached to 3 other C]
No oxidation except under extreme conditions
Iodoform reaction - (to be done again in aldehydes and ketones)
http://www.chemguide.co.uk/organicprops/alcohols/iodoform.html#top
Alcohols with the formula CH3CH(OH)R [ethanol if R = H] are oxidized by sodium iodate (I) to
CH3COR and therefore give a positive iodoform test.
In the formula CH3CH(OH)R: The alpha carbon is CH3. The alpha carbon is one carbon away from the
carbon with the functional group.
Iodoform is CHI3: Tri-iodomethane (fine yellow crystals with characteristic smell)
Stage 1: alcohol is oxidized to a carbonyl compound
Example: Ethanol in the presence of I2 / NaOH 
Ethanal
Stage 2: The 3 hydrogens on the alpha carbon are replaced with iodine
Example: ethanal in the presence of I2 / NaOH  Tri-iodo-ethanal
Stage 3: The excess base hydrolyses the molecule and causes the C-C bond to break, releasing
triiodomethane [CHI3].
Example: Tri-iodo-ethanal in the presence of OH-  Tri-iodomethane + the methoxide anion
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 3 OF 9
From Chemguide:
Summary of the reactions during the triiodomethane (iodoform) reaction
We will take the reagents as being iodine and sodium hydroxide solution.
(e)
(f) The structure of phenol
http://www.chemguide.co.uk/organicprops/phenol/background.html#top
Phenol will be look at again after the topic BENZENE
From Chemguide:
The structure of phenol
The simplest way to draw the structure of phenol is:
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 4 OF 9
There is an interaction between the delocalised electrons in the benzene ring and one of the
lone pairs on the oxygen atom. This has an important effect on both the properties of the ring
and of the -OH group.
One of the lone pairs on the oxygen overlaps with the delocalised ring electron system . . .
. . . giving a structure rather like this:
The donation of the oxygen's lone pair into the ring system increases the electron density
around the ring. That makes the ring much more reactive than it is in benzene itself.
It also helps to make the -OH group's hydrogen a lot more acidic than it is in alcohols.
The -OH group attached to the benzene ring in phenol has the effect of making the ring much
more reactive than it would otherwise be.
For example, as you will find below, phenol will react with a solution of bromine in water
(bromine water) in the cold and in the absence of any catalyst. It also reacts with dilute nitric
acid, whereas benzene itself needs a nitrating mixture of concentrated nitric acid and
concentrated sulphuric acid.
Reaction with bromine water
http://www.chemguide.co.uk/organicprops/phenol/ring.html#top
If bromine water is added to a solution of phenol in water, the bromine water is decolourised and
a white precipitate is formed which smells of antiseptic.
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 5 OF 9
The precipitate is 2,4,6-tribromophenol.
Notice the multiple substitution around the ring - into all the activated positions. (The 6- position
is, of course, just the same as the 2- position. Both are next door to the -OH group.)
Note: Bromine water is normally used as a test for a C=C double bond. The important difference with phenol is the
formation of a white precipitate as well as the bromine water being decolourised.
If you choose to follow this link, use the BACK button on your browser to return to this page.
Combustion of phenol
http://www.chemguide.co.uk/organicprops/phenol/other.html#top
Phenol burns in a plentiful supply of oxygen to give carbon dioxide and water.
However, for compounds containing benzene rings, combustion is hardly ever complete,
especially if they are burnt in air. The high proportion of carbon in phenol means that you need a
very high proportion of oxygen to phenol to get complete combustion. Look at the equation.
As a general rule, the hydrogen in a molecule tends to get what oxygen is available first, leaving
the carbon to form carbon itself, or carbon monoxide, if there isn't enough oxygen to go round.
Phenol tends to burn in air with an extremely smoky flame - full of carbon particles.
Esterification of phenol
http://www.chemguide.co.uk/organicprops/phenol/other.html#top
You will probably remember that you can make esters from alcohols by reacting them with
carboxylic acids. You might expect phenol to be similar.
However, unlike alcohols, phenol reacts so slowly with carboxylic acids that you normally react it
with acyl chlorides (acid chlorides) or acid anhydrides instead.
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 6 OF 9
Making esters from phenol using an acyl chloride
A typical acyl chloride is ethanoyl chloride, CH3COCl.
Phenol reacts with ethanoyl chloride at room temperature, although the reaction isn't as fast as
the one between ethanoyl chloride and an alcohol. Phenyl ethanoate is formed together with
hydrogen chloride gas.
Sometimes it is necessary to modify the phenol first to make the reaction faster.
For example, benzoyl chloride has the formula C6H5COCl. The -COCl group is attached directly
to a benzene ring. It is much less reactive than simple acyl chlorides like ethanoyl chloride.
In order to get a reasonably quick reaction with benzoyl chloride, the phenol is first converted
into sodium phenoxide by dissolving it in sodium hydroxide solution.
The phenoxide ion reacts more rapidly with benzoyl chloride than the original phenol does, but
even so you have to shake it with benzoyl chloride for about 15 minutes. Solid phenyl benzoate
is formed.
Making esters from phenol using an acid anhydride
A typical acid anhydride is ethanoic anhydride, (CH3CO)2O.
The reactions of acid anhydrides are slower than the corresponding reactions with acyl
chlorides, and you usually need to warm the mixture.
Again, you can react the phenol with sodium hydroxide solution first, producing the more
reactive phenoxide ion.
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 7 OF 9
If you simply use phenol and ethanoic anhydride, phenyl ethanoate is formed together with
ethanoic acid.
This reaction isn't important itself, but a very similar reaction is involved in the manufacture of
aspirin (covered in detail on another page - link below).
If the phenol is first converted into sodium phenoxide by adding sodium hydroxide solution, the
reaction is faster. Phenyl ethanoate is again formed, but this time the other product is sodium
ethanoate rather than ethanoic acid.
Properties of phenol as an acid
http://www.chemguide.co.uk/organicprops/phenol/acidity.html#top
With indicators
The pH of a typical dilute solution of phenol in water is likely to be around 5 - 6 (depending on its
concentration). That means that a very dilute solution isn't really acidic enough to turn litmus
paper fully red. Litmus paper is blue at pH 8 and red at pH 5. Anything in between is going to
show as some shade of "neutral".
With sodium hydroxide solution
Phenol reacts with sodium hydroxide solution to give a colourless solution containing sodium
phenoxide.
In this reaction, the hydrogen ion has been removed by the strongly basic hydroxide ion in the
sodium hydroxide solution.
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 8 OF 9
With sodium carbonate or sodium hydrogencarbonate
Phenol isn't acidic enough to react with either of these. Or, looked at another way, the carbonate
and hydrogencarbonate ions aren't strong enough bases to take a hydrogen ion from the
phenol.
Unlike the majority of acids, phenol doesn't give carbon dioxide when you mix it with one of
these.
This lack of reaction is actually useful. You can recognise phenol because:
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It is fairly insoluble in water.
It reacts with sodium hydroxide solution to give a colourless solution (and therefore must
be acidic).
It doesn't react with sodium carbonate or hydrogencarbonate solutions (and so must be
only very weakly acidic).
With metallic sodium
Acids react with the more reactive metals to give hydrogen gas. Phenol is no exception - the
only difference is the slow reaction because phenol is such a weak acid.
Phenol is warmed in a dry tube until it is molten, and a small piece of sodium added. There is
some fizzing as hydrogen gas is given off. The mixture left in the tube will contain sodium
phenoxide.
CAMPION
J. MARSHALL
CAPE CHEM
U2 M1
ORGANIC – ALCOHOLS & PHENOL
PAGE 9 OF 9