Transcript File

PowerPoint Lecture Presentation
by
J. David Robertson
University of Missouri
Organic Chemistry II :
Reactions of the
Functional Groups
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Revision of Functional Groups
Family
Alkane
Alkene
Alkyne
Haloalkane
Alcohol
Aldehyde
Ketone
Carboxylic acid
Ester
C—H
and
C—C
bonds
Ether
Common Classes of Organic Reactions
A) Substitution Reactions
In substitution reactions , one atom or a group of atoms in a
substance is replaced by another atom or a group of atom from
another substance. Example:
CH3Cl + OH—  CH3OH + Cl—
B) Elimination Reactions
Reactions in which adjacent atoms are removed or ‘eliminated’
from a molecule with the formation of a multiple bond and
release of a small molecule are called elimination reactions. A
general example is:
Common Classes of Organic Reactions
C) Addition Reactions
A reaction in which the component of a species A—B are
added to adjacent atoms across a C—C multiple bond is called
addition reaction. A general example is:
D) Radical Reactions
Radical reactions occur in three stages: initiation; propagation
& termination.
1. Initiation step produces an appreciable number of radical atoms.
2. During propagation step, the radical attacks a neutral molecule,
lengthening of the molecule’s chain can be done in this process.
3. There are three possible termination steps: the combination of
two radical atoms; the combination of two radical molecules or
the combination of a radical atom with a radical molecule.
Common Classes of Organic Reactions
E) Oxidation-Reduction Reactions
Oxidation-reduction reactions inorganic chemistry can often be identified
by changes in the number of O atoms at a particular position in the
hydrocarbon skeleton or in the number of bonds between C and O at that
position.
Conversely, an increase in the number of H atoms is often an indication of a
reduction.
Functional
Group:
Oxidation
state of C:
H3C—H
Alkane
−4
H3C—OH
Alcohol
−2
H2C = O
Aldehyde
0
HCOOH
Carboxylic acid
+2
Reactions of Alkane
1) Combustion
Complete combustion (given sufficient oxygen) produces carbon
dioxide and water.
CH4 (g) + 2 O2 (g) → CO2 (g) + 2H2O (l)
ΔH0 = -890.4 kJ
2C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(l)
ΔHo = –2855 kJ
Incomplete combustion (where there isn't enough oxygen
present) can lead to the formation of carbon (soot) or carbon
monoxide.
2 C2H6(g) + 5 O2(g)  4 CO(g) + 6 H2O(l)
2 C2H6(g) + 3 O2(g)  4 C(s) + 6 H2O(l)
Carbon dioxide contributes to global warming while carbon
monoxide is toxic; hemoglobin binds to carbon monoxide in
preference to oxygen causing suffocation and even death.
24.2
Reactions of Alkane
2) Halogenation
Alkanes react with halogens in a so-called free radical halogenation reaction
(substitution reaction). The hydrogen atoms of the alkane are progressively
replaced by halogen atoms.
R—H + Br2  RBr + HBr
Free-radicals are the reactive species that participate in the reaction, which
usually leads to a mixture of products. The reaction is highly exothermic, and
can lead to an explosion.
There are three steps:
• Initiation: the halogen radicals form by homolysis. Usually, energy in the form
of heat or light is required.
• Chain reaction or Propagation: he halogen radical abstracts a hydrogen from
the alkane to give an alkyl radical. This reacts further.
• Chain termination: this is the step where the radicals recombine.
24.2
Initiation:
Cl• + Cl•
Cl2 + energy
Propagation:
H
Cl• + H
C
H
H
H
•C
H
+ HCl
H
Chain termination:
H
H
C • + Cl
H
H
Cl
H
C
H
Cl + Cl•
INTRODUCING HALOGENOALKANES
(haloalkanes or alkyl halides)
Halogenoalkanes are compounds in which one or more hydrogen
atoms in an alkane have been replaced by halogen atoms (fluorine,
chlorine, bromine or iodine).
Elimination reaction between halogenoalkanes and hydroxide
ions:
Halogenoalkanes also undergo elimination reactions in the
presence of sodium or potassium hydroxide.
Reactions of Alkane
3) Cracking
When heated at high temperatures in the absence of air, alkanes
can "crack," meaning that they break up into smaller molecules.
Cracking can be achieved by using high pressures and temperatures
without a catalyst, or lower temperatures and pressures in the
presence of a catalyst.
The hydrocarbon molecules are broken up in a fairly random way
to produce mixtures of smaller hydrocarbons, some of which have
carbon-carbon double bonds. One possible reaction involving the
hydrocarbon C15H32 might be:
Thermal cracking
In thermal cracking, high temperatures (typically in the range of
450°C to 750°C) and pressures (up to about 70 atmospheres) are
used to break the large hydrocarbons into smaller ones. Thermal
cracking gives mixtures of products containing high proportions of
hydrocarbons with double bonds - alkenes.
The cracking of methane gives finely powered carbon and
hydrogen gas.
high temperatures
CH4
C + 2H2
The controlled cracking of ethane gives ethene ("ethylene"),
which is an important raw material in the organic chemicals
industry, used to make polyethylene plastics, ethyl alcohol, and
ethylene gycol (an antifreeze).
high temperatures
CH3CH3
ethane
CH2=CH2 + H2
ethene
Catalytic cracking
Modern cracking uses zeolites as the catalyst. These are complex
aluminosilicates, and are large lattices of aluminium, silicon and
oxygen atoms carrying a negative charge.
The alkane is brought into contact with the catalyst at a
temperature of about 500°C and moderately low pressures.
The zeolites used in catalytic cracking are
chosen to give high percentages of
hydrocarbons with between 5 and 10
carbon atoms - particularly useful for
petrol (gasoline). It also produces high
proportions of branched alkanes and
aromatic hydrocarbons like benzene.
Reactions of Alkene
1) Hydrogenation of alkene:
Alkene reacts with hydrogen in the presence of a finely divided
metal catalyst (nickel, palladium or platinum) at a temperature of
about 150°C. Alkane is produced.
Margarine manufacture:
Some margarine is made by hydrogenating
carbon-carbon double bonds in animal or
vegetable fats and oils. You can recognise the
presence of this in foods because the ingredients
list will include words showing that it contains
"hydrogenated vegetable oils" or "hydrogenated
fats".
24.2
Reactions of Alkene
2) Hydration of alkenes:
Ethanol is manufactured by reacting ethene with steam. The
catalyst used is solid silicon dioxide coated with phosphoric(V) acid.
The reaction is reversible.
3) Halogenation of alkenes:
Halogenation of alkene is addition reaction. For example,
bromine adds to give 1,2-dibromoethane.
Chlorine reacts faster than bromine, but the chemistry is similar.
Iodine reacts much, much more slowly, but again the chemistry is
similar.
Reactions of Alkene
4) Hydrohalogenation of alkenes:
All alkenes undergo addition reactions with the hydrogen halides. A hydrogen
atom joins to one of the carbon atoms originally in the double bond, and a
halogen atom to the other.
For example, with ethene and hydrogen chloride, you get chloroethane:
If HCl adds to an unsymmetrical alkene like propene, there are two possible ways
it could add. However, in practice, there is only one major product.
This is in line with
Markovnikov's Rule which says:
When a compound HX is added
to an unsymmetrical alkene, the
hydrogen becomes attached to
the carbon with the most
hydrogens attached to it already.
Important!!!!
Reactions of Alkene
5) Polymerization of alkenes:
Polymerisation of alkenes produce polymers like poly(ethene) (usually known as
polythene, and sometimes as polyethylene), poly(propene) (old name:
polypropylene), PVC and PTFE.
The reaction is an example of addition polymerisation in which two or more
molecules join together to give a single product. During the polymerisation of
ethene, thousands of ethene molecules join together to make poly(ethene) commonly called polythene.
Poly(chloroethene) is commonly known by the initials of its old name, PVC.
Poly(chloroethene) is made by polymerising chloroethene, CH2=CHCl.
Reactions of Alkene
6) Benzene: Electrophilic Substitution
Benzene, C6H6, is a planar molecule containing a ring of six carbon
atoms each with a hydrogen atom attached. There are delocalized
electrons above and below the plane of the ring.
The presence of the delocalized electrons
makes benzene particularly stable.
Benzene resists addition reactions because
that would involve breaking the
delocalization and losing that stability.
Benzene is represented by this symbol,
where the circle represents the
delocalized electrons, and each corner
of the hexagon has a carbon atom with
a hydrogen attached.
Because of the delocalized electrons exposed above and below the plane of the
rest of the molecule, benzene is obviously going to be highly attractive to
electrophiles - species which seek after electron rich areas in other molecules.
The electrophile will either be a positive ion, or the slightly positive end of a
polar molecule.
Suppose the electrophile is a positive ion X+.
Two of the electrons in the delocalised system are attracted towards the X+ and
form a bond with it. This has the effect of breaking the delocalisation, although
not completely.
Example: nitration of benzene
H2SO4 conc.
The concentrated sulphuric acid is acting as a catalyst.
Step by step:
a)
b)
c)
Reactions of Alcohol
1) Dehydration of alcohols to produce alkene:
The dehydration of ethanol gives ethene. This is a simple way of making
gaseous alkenes like ethene. If ethanol vapour is passed over heated
aluminium oxide powder, the ethanol is essentially cracked to give ethene
and water vapour
Dehydration may also be carried out by heating the alcohol with excess
concentrated H2SO4 or concentrated H3PO4 at 170-180oC.
24.3
Reactions of Alcohol
2) Oxidation of alcohols:
Oxidation of alcohols is done using acidified sodium or potassium
dichromate(VI) solution. This reaction is used to make aldehydes, ketones and
carboxylic acids, and as a way of distinguishing between primary, secondary
and tertiary alcohols.
The oxidising agent used in these reactions is normally a solution of sodium or
potassium dichromate(VI) acidified with dilute sulphuric acid. If oxidation
occurs, the orange solution containing the dichromate(VI) ions is reduced to a
green solution containing chromium(III) ions.
If the alcohol's hydroxyl group is at the end of a
carbon atom chain, an oxidation reaction
produces either a carboxylic acid or an aldehyde.
If the hydroxyl group is attached in the middle of
a straight carbon atom chain, an oxidation
reaction produces a ketone.
24.3
Primary alcohols
Primary alcohols can be oxidised to either aldehydes or carboxylic acids
depending on the reaction conditions. In the case of the formation of carboxylic
acids, the alcohol is first oxidised to an aldehyde which is then oxidised further
to the acid.
Full oxidation to carboxylic acids
You need to use an excess of the oxidising agent and make sure that the
aldehyde formed as the half-way product stays in the mixture. The alcohol is
heated under reflux with an excess of the oxidising agent. When the reaction is
complete, the carboxylic acid is distilled off.
The full equation for the oxidation of ethanol to ethanoic acid is:
Secondary alcohols
Secondary alcohols are
oxidised to ketones. For
example, reaction using
propan-2-ol will produced
propanone .
Carboxylic Acids
The pH of carboxylic acid solutions
The pH depends on both the concentration of the acid and how
easily it loses hydrogen ions from the -COOH group.
Ethanoic acid is typical of the acids where the -COOH group is
attached to a simple alkyl group. Typical lab solutions have pH's
in the 2 - 3 range, depending on their concentrations.
This reaction is reversible and, in the case of ethanoic acid, no
more than about 1% of the acid has reacted to form ions at any
one time. (This is a rough-and-ready figure and varies with the
concentration of the solution.)
24.2
Reactions of Carboxylic Acids
With metals:
Carboxylic acids react with the more reactive metals to produce a
salt and hydrogen gas. The reactions are just the same as with acids
like hydrochloric acid, except they tend to be rather slower.
2 CH3COOH + Mg → (CH3COO)2Mg + H2
With metal hydroxides:
These are simple neutralization reactions and are just the same as
any other reaction in which hydrogen ions from an acid react with
hydroxide ions.
CH3COOH + NaOH → CH3COONa + H2O
24.2
Reactions of Carboxylic Acids
With ammonia
Ethanoic acid reacts with ammonia in exactly the same way as any
other acid does. It transfers a hydrogen ion to the lone pair on the
nitrogen of the ammonia and forms an ammonium ion.
Reaction between aqueous solutions of ethanoic acid and ammonia
will produce a colourless solution of ammonium ethanoate.
CH3COOH + NH3 → CH3COONH4
CH3COOH + NH4OH → CH3COONH4 + H2O
Reactions of Carboxylic Acids
With amines
Amines are compounds in which one or more of the hydrogen atoms in an
ammonia molecule have been replaced by a hydrocarbon group such as an alkyl
group. Replacing the hydrogens still leaves the lone pair on the nitrogen
unchanged - and it is the lone pair on the nitrogen that gives ammonia its basic
properties. Amines will therefore behave much the same as ammonia in all cases
where the lone pair is involved.
Primary amines are the compounds where only one of the hydrogen atoms has
been replaced. Example of primary amines:
Amines will react with carboxylic acids the same way as ammonia does. For
example, ethanoic acid reacts with methylamine to produce a colourless solution
of the salt methylammonium ethanoate.
Reactions of Carboxylic Acids
Esterification - the reaction between alcohols and carboxylic acids to make
esters.
Esters are produced when carboxylic acids are heated with alcohols in the
presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid.
The esterification reaction is both slow and reversible. The equation for the
reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be
the same or different) is:
Reactions of Carboxylic Acids
Esters can react with amines to produce amides and alcohols.
Amides are derived from carboxylic acids. A carboxylic acid contains the -COOH
group, and in an amide the -OH part of that group is replaced by an -NH2 group.
The three simplest amides are:
HCONH2
Methanamide
CH3CONH2
Ethanamide (acetamide)
CH3CH2CONH2
Propanamide
(Notice that in each case, the
name is derived from the acid by
replacing the "oic acid" ending
by "amide".)
Unlike amines, the amides are non-basic even though they have the NH2 group
in simple amides. This is because of the presence of O atom in the carbonyl
group, which is very electronegative. This tightens the electrons on N so that it
is unable to accept a proton.
Reactions of Carboxylic Acids
Reaction between an ester and an amine is shown below:
Ester
Amide
Amine
Alcohol
Aldehyde & Ketone
Aldehydes and ketones are simple compounds which contain a carbonyl group a carbon-oxygen double bond.
An aldehyde differs from a ketone by having a hydrogen atom attached to the
carbonyl group. This makes the aldehydes very easy to oxidise. Ketones don't
have that hydrogen atom and are resistant to oxidation.
Therefore, the major difference between an aldehyde and a ketone is:
Aldehydes are easily oxidised by all sorts of different oxidising agents but
ketones aren't.
Reactions of Aldehydes & Ketones
1) Addition of hydrogen cyanide to aldehydes and ketones
Hydrogen cyanide adds across the carbon-oxygen double bond in aldehydes
and ketones to produce compounds known as hydroxynitriles.
ethanal
propanone
2-hydroxypropanenitrile
2-hydroxy-2-methylpropanenitrile
Reactions of Aldehydes & Ketones
2) Reduction of aldehydes and ketones by lithium aluminium
hydride and sodium borohydride.
The formulae of the two reducing agents are LiAlH4 and NaBH4. Their structures
are:
The reduction of an aldehyde:
The same organic product will be obtained whether you use LiAlH4 and NaBH4.
ethanal
ethanol
Notice that this is a simplified equation – [H] means "hydrogen from a
reducing agent".
In general terms, reduction of an aldehyde leads to a primary alcohol.
Reactions of Aldehydes & Ketones
The reduction of a ketone:
The same organic product will be obtained whether you use LiAlH4 and NaBH4.
propanone
Propan-2-ol
Reduction of a ketone leads to a secondary alcohol.
Reactions of Aldehydes & Ketones
3) Oxidation of aldehydes and ketones.
These reactions help to distinguish between aldehydes and ketones. The
oxidising agents used are acidified potassium dichromate(VI) solution, Tollens'
reagent, Fehling's solution and Benedict's solution.
What is formed when aldehydes are oxidised?
It depends on whether the reaction is done under acidic or alkaline
conditions. Under acidic conditions, the aldehyde is oxidised to a carboxylic
acid. Under alkaline conditions, this couldn't form because it would react
with the alkali. A salt is formed instead.
Using acidified potassium dichromate(VI) solution
A small amount of potassium dichromate(VI) solution is acidified with dilute
sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing
happens in the cold, the mixture is warmed gently for a couple of minutes - for
example, in a beaker of hot water.
ketone
No change in the orange solution.
aldehyde
Orange solution turns green.
The orange dichromate(VI) ions have been reduced to green chromium(III)
ions by the aldehyde. In turn the aldehyde is oxidised to the corresponding
carboxylic acid.
Using Tollens' reagent (the silver mirror test)
Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+.
This is made from silver(I) nitrate solution. Add a drop of sodium hydroxide
solution to give a precipitate of silver(I) oxide, and then add just enough dilute
ammonia solution to redissolve the precipitate.
To carry out the test, add a few drops of the aldehyde or ketone to the freshly
prepared reagent, and warm gently in a hot water bath for a few minutes.
ketone
No change in the colourless solution.
The colourless solution produces a grey precipitate
of silver, or a silver mirror on the test tube.
Aldehydes reduce the diamminesilver(I) ion to
aldehyde
metallic silver. Because the solution is alkaline, the
aldehyde itself is oxidised to a salt of the
corresponding carboxylic acid.
Using Fehling's solution or Benedict's solution
Fehling's solution and Benedict's solution are variants of essentially the same
thing. Both contain complexed copper(II) ions in an alkaline solution.
Fehling's solution contains copper(II) ions complexed with tartrate ions in
sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions
prevents precipitation of copper(II) hydroxide.
Benedict's solution contains copper(II) ions complexed with citrate ions in
sodium carbonate solution. Again, complexing the copper(II) ions prevents the
formation of a precipitate - this time of copper(II) carbonate.
Both solutions are used in the same way. A few drops of the aldehyde or
ketone are added to the reagent, and the mixture is warmed gently in a hot
water bath for a few minutes.
ketone
No change in the blue solution.
aldehyde
The blue solution produces a dark red precipitate of
copper(I) oxide.
Aldehydes reduce the complexed copper(II) ion to copper(I) oxide. Because the
solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding
carboxylic acid.