ORGANIC CHEMISTRY

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ORGANIC CHEMISTRY

• AS Module 3

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NAMING 1

• Look for the longest carbon chain. This gives
  the base name for your molecule
• 1 C methan- 2 C ethan-
• 3 C propan- 4 C butan-
• 5 C pentan- 6 C hexan-

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E.g. Name me
3-methylhexane
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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
• CH2CH2
• CH3-CN

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-CHO and CH2OH

• Aldehydes have the -CHO grouping.E.g.
  propanal
• Alcohols have the -CH2OH groupingE.g.
  propan-2-ol

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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.

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E.g. Name me
5-chloropentan-2-ol
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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.

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E.g. Name me
3,4,4-trichloropentanal
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ISOMERISMSTRUCTURAL 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)

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CHAIN ISOMERISM

• Structural isomers with different carbon chains
• E.g. for C5H12

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POSITION ISOMERISM

• Structural isomers with different positions for
  the functional group
• E.g. for C3H7OH

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FUNCTIONAL GROUP ISOMERS

• Structural isomers with different functional
  groups
• E.g. for C4H8O

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HOMOLOGOUS SERIES

• Same
• Functional group
• Chemical reactions
• General formula
• Gradually changing physical properties

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ALKANES - SOURCE

• From? Crude Oil
• By?
• 1. Fractional DistillationLearn fractions,
  order of B.Pts. uses
• 2. Cracking Be able to write an equationE.g.
  C14H30 can be cracked to give octane and ethene
  only
• C14H30 C8H18 3C2H4

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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

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ALKANES PHYSICAL PROPERTIES

• Symmetrical non-polar molecules
• \ Intermolecular forces?
• Weak Van der Waals
• \ 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

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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

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FREE RADICAL SUBSTITUTION 1
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FREE RADICAL SUBSTITUTION 2
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HALOALKANES

• Polar molecules. Why?
• So dipole-dipole forces and slightly higher
  M.Pts. etc. than the alkanes
• Because of the bond polarity dCBrd-
• The carbon is attacked by nucleophiles (?)

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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-

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NUCLEOPHILIC SUBSTITUTION 2
Note the nucleophilic attack by the CN- ion. The
lone pair on the C attacks
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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.
  Dont forget charges on ions

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LOOK AGAIN!
The examiner is very strict about curly arrows in
mechanismsNote an extra C is added to the chain.
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NitrilesUseful 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

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NUCLEOPHILIC SUBSTITUTION 3

• Ammonia as a nucleophile needs 2 stages

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ELIMINATION FROM A HALOALKANE

• Refluxing a haloalkane with KOH dissolved in
  ethanol produces an alkene. E.g
• CH3CH2CH2Br OH- CH3CHCH2 H2O Br-
• Note The change of solvent leads to a different
  reactionThe OH- acts as a base here (rather than
  as a nucleophile) as it picks up a proton.

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Elimination Mechanism
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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.

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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

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1º 2º 3º Alcohols

• 1º2º3º

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Reactions of Alcohols 1Oxidation

• Oxidant of choice
• Acidified potassium dichromate
• Colour change
• Orange to green when it oxidises something

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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

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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 CH 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

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(Non) Oxidation of 3º Alcohols

• Note that there are no CH bonds on the carbon
  attached to the hydroxy group.
• Therefore a tertiary alcohol will not be oxidised.

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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 alcoholIf it does not
  oxidise it is tertiary
• 2. If it can be oxidisedTest the product of
  oxidation to see whether it is an aldehyde

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Tests for aldehydes

• Both Tollens Fehlings can be used.Quote one
  accurately
• TollensWarming 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.

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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 H2O mix of
  CH3CH2CHCH2and CH3CHCHCH3depending on which
  side of the COH the proton is removed from.

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Elimination Mechanism
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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

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Geometrical Isomerism

• Cis but-2-ene

• Trans but-2-ene

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Reactions of Alkenes

• The CC 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

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Electrophilic Addition Reactions

• Alkenes react with
• HBr (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).

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Electrophilic Addition Mechanism 1
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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
• CH3CHCH2 H-Br CH3CH(Br)CH3Very little of
  the 1-bromopropane is produced

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Addition to Unsymmetrical Alkenes 2

• Reason?
• The 2º carbocation produced on the way to
  2-bromopropane CH3CHCH3is more stable than
  the 1º carbocation produced on the way to
  1-bromopropane CH3CH2CH2
• Order of stability of carbocations3º gt 2º gt
  1º

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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
  thatThe d part of the electrophile attaches to
  the carbon of the double bond which has most
  hydrogens(NOT an explanation!)

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Electrophilic Addition Mechanism 2
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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 RCHCH2 H2 RCH2-CH3

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Polymerisation of Alkenes

• Also an addition reaction
• Mechanism free radical
• n CH2CH2 --(-CH2CH2-)n-
• Polyethene
• n CH2CHCl --(-CH2CHCl-)n-
• Polychloroethene or PVC
• Polystyrene from styrene CH2CHC6H5 ?

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Epoxyethane 1

• A very useful compound made from ethene

Ag catalyst. Heat. In oxygen or air
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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 CC 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
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EPOXYETHANE WATER
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EPOXYETHANE ALCOHOLS