Transcript document

ORGANIC CHEMISTRY
AS Module 3
NAMING 1
• Look for the longest carbon chain. This
gives the base name for your molecule:
• 1 C = methan2 C = ethan• 3 C = propan-
4 C = butan-
• 5 C = pentan-
6 C = hexan-
E.g. Name me:
H3C
CH 2
H2C
CH
H3C
CH 2
CH3
FUNCTIONAL GROUPS
• Functional group =
an atom or group in
the molecule that
determines the
chemical properties
• Recognise the reactive
group in the molecule.
• When you know the
functional group you
can predict the
reactions of the
molecule
• E.g. CH3CH2-OH
•
CH3CH2-Br
•
CH3-CHO
•
CH3-COOH
•
CH2=CH2
•
CH3-CN
-CHO and –CH2OH
• Aldehydes have the
-CHO grouping.
E.g. propanal:
H3C
CH 2
C
H
• Alcohols have the
-CH2OH grouping
E.g. propan-2-ol
OH
H3C
HC
CH 3
O
NAMING 2
• Identify the functional groups/substituents
and number the carbons in the chain,
starting from one end, to keep the number
of the functional group or substituent as low
as possible.
• Remember that a functional group gets
priority for low numbering.
E.g. Name me:
Cl
CH 2
H2C
HO
CH 2
CH
CH3
NAMING 3
• Substituents are named in front of the base
name.
• Remember di- = 2, tri- = 3 etc. if there is
more than one of the particular substituent
attached.
• And remember to specify positions on the
chain.
E.g. Name me:
Cl
H
CH
C
O
CH3
C
CH2
Cl
Cl
ISOMERISM
STRUCTURAL & STEREO
• STRUCTURAL =
Same molecular
formula,
atoms(groups) bonded
in different places:
– Chain
– Position
– Functional group
• STEREO = Same
molecular formula and
structure,
atoms(groups)
arranged differently in
space:
– Geometrical (cis/trans)
– Optical (next year)
CHAIN ISOMERISM
• Structural isomers with different carbon chains:
• E.g. for C5H12:
CH3
H3C
CH 2
H3C
C
H3C HC
CH 2 CH 2
CH3
CH3
CH 2 CH3
H3C
CH3
POSITION ISOMERISM
• Structural isomers with different positions
for the functional group:
• E.g. for C3H7OH:
OH
H3C
CH 2
CH 2 OH
H3C
HC
CH3
FUNCTIONAL GROUP
ISOMERS
• Structural isomers with different functional
groups:
• E.g. for C4H8O:
O
H3C
CH2
H
CH2 C
H3C
C
CH2 CH3
O
HOMOLOGOUS SERIES
•
•
•
•
Same:
Functional group
Chemical reactions
General formula
&
• Gradually changing physical properties
ALKANES - SOURCE
• From?
Crude Oil
• By?
• 1. Fractional Distillation
Learn fractions, order of B.Pts. & uses
• 2. Cracking
Be able to write an equation
E.g. C14H30 can be cracked to give octane
and ethene only
• C14H30  C8H18 + 3C2H4
2 TYPES OF CRACKING
•
•
•
•
•
THERMAL
HIGH T + HIGH P
~800ºC
FREE RADICAL
PRODUCES MORE
ALKENE
MOLECULES FOR
PETROCHEMICALS
•
•
•
•
•
CATALYTIC
LOWER T + CAT.
~450ºC ZEOLITE
VIA CARBOCATION
TO GIVE MORE
SMALL ALKANES
FOR PETROL
ALKANES – PHYSICAL
PROPERTIES
• Symmetrical non-polar molecules
 \ Intermolecular forces?
• Weak Van der Waal’s
 \ Low M.Pts. & B.Pts. compared to most
covalent molecules of similar Mr
• Also insoluble in water as they cannot form
hydrogen bonds with water molecules
ALKANES - REACTIONS
•
•
•
•
•
•
Saturated
Hydrocarbons
Unreactive except for 2 major reactions:
Combustion: E.g. butane
C4H10 + ?O2
4CO2 + 5H2O
Substitution by a halogen e.g. chlorine
FREE RADICAL
SUBSTITUTION 1
INITIATION:
Cl
Cl
Cl
+
Cl
PROPAGATION:
CH4
CH3
+
+
Cl
+
CH3
Cl
Cl
H3C
Cl
+
HCl
Cl
FREE RADICAL
SUBSTITUTION 2
TERMINATION:
CH3
CH3
Cl
+
+
+
CH3
H3C
CH3
Cl
H3C
Cl
Cl
Cl
Cl
HALOALKANES
• Polar molecules. Why?
• So dipole-dipole forces and slightly higher
M.Pts. etc. than the alkanes
• Because of the bond polarity:
d+C—Brd• The carbon is attacked by nucleophiles (?)
Nucleophilic Substitution 1
• Haloalkanes can be converted into:
• Alcohols
(NaOH(aq) + heat)
CH3Br + OH- CH3OH +
Br• Amines
(XS conc. NH3(aq) + heat)
CH3Br + 2NH3 CH3NH2 + NH4+ +
Br• Nitriles
(KCN(ethanol) + heat)
CH3Br + CN- CH3CN +
Br-
NUCLEOPHILIC
SUBSTITUTION 2
N
xx -
C
H3C
H3C
C
N
+
Cl
Cl
Note the nucleophilic attack by the CNion. The lone pair on the C attacks
-
CURLY ARROWS
• They show movement of an electron pair
• They start on a lone pair or on a covalent
bond
• Remember to show clearly the molecule
or ion produced after each stage of the
mechanism. Don’t forget charges on ions
LOOK AGAIN!
N
xx -
C
H3C
H3C
C
N
+
Cl
The examiner is very strict about curly
arrows in mechanisms
Note: an extra C is added to the chain.
Cl
-
Nitriles
Useful Intermediates
• Can be converted to
carboxylic acids:
• Reflux with dilute acid
(or alkali)
• E.g.
CH3CN + 2H2O 
CH3COO- + NH4+
• HYDROLYSIS
• Can be converted to
amines:
• Heat in hydrogen with
a Ni catalyst
• E.g.
CH3CN + 2H2 
CH3CH2NH2
• REDUCTION
NUCLEOPHILIC
SUBSTITUTION 3
• Ammonia as a nucleophile needs 2 stages:
H3C
xx
H
Cl
H3C
N
H H
+
NH2
H
xx
H
H3C
NH2
+
+
NH4
N
H H
+
Cl
-
ELIMINATION FROM A
HALOALKANE
• Refluxing a haloalkane with KOH dissolved
in ethanol produces an alkene. E.g:
• CH3CH2CH2Br + OH- 
CH3CH=CH2 + H2O + Br• Note: The change of solvent leads to a
different reaction
The OH- acts as a base here (rather than as
a nucleophile) as it picks up a proton.
Elimination Mechanism
Br
CH
CH3
H2C
H
xx -
HO
ALCOHOLS
• Homologous series?
• Functional group –OH
• Thus high M.Pts & B.Pts for typical
covalent molecules, because?
• Can form hydrogen bonds between
molecules
• Thus the smaller alcohols also mix with
water.
3 TYPES OF ALCOHOL
•
•
•
•
•
Not Whiskey, Beer, and Wine!
Primary 1º
Secondary 2º
Tertiary 3º
According to the no. of Carbons attached to
the Carbon with –OH attached to it:
1º 2º & 3º Alcohols
CH 2 CH 2
• 1º
HO
2º
CH 2 CH3
H3C
CH 2
CH
OH
H3C
3º
H3C
C
H3C
CH3
OH
Reactions of Alcohols 1
Oxidation
•
•
•
•
Oxidant of choice:
Acidified potassium dichromate
Colour change:
Orange to green when it oxidises something
Oxidation of 1º Alcohols
• 1º gives an aldehyde on heating and
distilling off the product straight away:
• CH3CH2OH + [O]  CH3CHO + H2O
• But refluxing the alcohol + oxidant gives
the acid as the aldehyde is oxidised:
• CH3CHO + [O]  CH3COOH
Oxidation of 2º Alcohols
• Here the oxidant will only produce the
ketone:
• CH3CH(OH)CH3 + [O] 
CH3COCH3 + H2O
• Note that the extent of oxidation depends on
how many C—H bonds can be broken
during the oxidation. The carbon chain
does not break unless the oxidation is very
vigorous i.e. combustion?
• CH3CH2OH + ?O2  2CO2 + 3H2O
(Non) Oxidation of 3º Alcohols
H3C
C
H3C
CH3
OH
• Note that there are no
C—H bonds on the
carbon attached to the
hydroxy group.
• Therefore a tertiary
alcohol will not be
oxidised.
Identifying Alcohols
• The fact that the alcohols respond
differently to oxidation gives us a simple
sequence of tests to identify the type:
• 1. Try oxidation of the alcohol
If it does not oxidise it is tertiary
• 2. If it can be oxidised:
Test the product of oxidation to see whether
it is an aldehyde:
Tests for aldehydes
• Both Tollens & Fehlings can be used.
Quote one accurately:
• Tollens:
Warming an aldehyde with Tollens causes
the colourless soln. to give a silver mirror
• Fehlings:
Warming an aldehyde with Fehlings causes
the blue soln. to give a red/brown ppt.
Elimination from Alcohols
• Heating an alcohol to 170ºC with conc.
H2SO4 produces an alkene as a water
molecule is eliminated.
• The acid acts as a catalyst
• CH3CH2CH(OH)CH3
 H 2O +
mix of CH3CH2CH=CH2
and
CH3CH=CHCH3
depending on which side of the C—OH the
proton is removed from.
Elimination Mechanism
H3C
H3C
CH 2
CH
CH 2
CH
CH3
H2O
Hxx
O
CH3
+
+
H
H3C
CH 2
CH
+
H
+
H3C
CH 2
CH
+
+
CH 2
H
CH2
H2O
ALKENES
• Homologous series?
• Non-polar Hydrocarbons \ type of
intermolecular forces?
• Van der Waals
\ low M.Pts. Etc. compared to alcohols and
immiscible with water.
• Exhibit a form of stereoisomerism called
Geometrical since there is no free rotation
about the double bond:
Geometrical Isomerism
• Cis but-2-ene
H3C
CH 3
C
H
• Trans but-2-ene
H
C
CH3
C
H
H3C
C
H
Reactions of Alkenes
• The C=C double bond is very reactive since
it is a centre of electron density.
One of the bonds is weaker than the other
and this breaks open on reaction leaving the
basic carbon chain intact.
• Thus alkenes undergo addition reactions
and are attacked by electrophiles i.e?
• ELECTROPHILIC ADDITION
Electrophilic Addition
Reactions
• Alkenes react with:
• H—Br (or other hydrogen halides)
• Br2 (a good test for alkenes as the brown
colour of the bromine quickly fades to
colourless)
• Conc. H2SO4 (if the product is warmed with
water an alcohol can be produced).
Electrophilic Addition
Mechanism 1
CH 2 Br
H2C
CH2
+
H2C
Br
Br
xx
Br
Br
CH 2
H2C
Br
-
Addition to Unsymmetrical
Alkenes 1
• When an unsymmetrical molecule like H-Br
is added to an unsymmetrical alkene like
propene, two products are possible but only
one is produced in any quantity:
• CH3CH=CH2 + H-Br  CH3CH(Br)CH3
Very little of the 1-bromopropane is
produced:
Addition to Unsymmetrical
Alkenes 2
• Reason?
• The 2º carbocation produced on the way to
+
2-bromopropane: CH3CHCH3
is more stable than the 1º carbocation
produced on the way to 1-bromopropane:
CH3CH2CH2+
• Order of stability of carbocations:
3º > 2º > 1º
Addition to Unsymmetrical
Alkenes 3
• Remember to draw the carbocations when
discussing stabilities
• In order to write correct equations, if you
are not asked for the mechanism, just
remember that:
The d+ part of the electrophile attaches to
the carbon of the double bond which has
most hydrogens
(NOT an explanation!)
Electrophilic Addition
Mechanism 2
CH 3
H2C
CH
H
H3C
CH3
CH
+
Br
xx
Br
Br
CH
H3C
CH3
-
Hydrogenation of Alkenes
• Alkene + Hydrogen + Heat with Ni catalyst
• Used to convert Unsaturated(?) vegetable
oils into more saturated margarine.
• The fewer the double bonds the harder the
margarine.
• E.g: R—CH=CH2 + H2  R—CH2-CH3
Polymerisation of Alkenes
•
•
•
•
•
•
•
Also an addition reaction:
Mechanism = free radical
n CH2=CH2  --(-CH2—CH2-)nPolyethene
n CH2=CHCl  --(-CH2—CHCl-)nPolychloroethene or PVC
Polystyrene from styrene CH2=CHC6H5 ?
Epoxyethane 1
• A very useful compound made from ethene
H2C
CH2
+
1/2 O2
H2 C
CH 2
O
Ag catalyst. Heat. In oxygen or air
Epoxyethane is very reactive because of the very strained 3
membered ring structure.
The bonds in the ring are forced to be at 60º to each other rather
than the usual 109½º for tetrahedral and hence one of the C—C
bonds breaks open easily (rather like an alkene)
Thus epoxyethane reacts easily with water and with alcohols:
(Warming with dilute acid catalyst).
With excess water ethane-1,2-diol is formed – used as antifreeze
and as a raw material for making polyesters.
With less water several epoxyethane molecules can add on to
form polymeric polyethene glycols – uses?
Similarly for the alcohol reactions
MAKE SURE YOU CAN WRITE THE EQUATIONS
EPOXYETHANE + WATER
HO
H2 C
CH 2
+
H2O
CH 2 CH 2
O
n
H2C
OH
CH 2
O
+
H2O
HO
(CH 2CH 2O)nH
EPOXYETHANE +
ALCOHOLS
H2C
CH2
+
H3C
OH
+
H3C
OH
CH3OCH2CH2OH
O
n
H2C
CH2
O
CH3O (CH2CH2O)nH