AS AQA Chemistry Unit 2 Organics
- Created by: chou
- Created on: 28-05-13 15:35
HALOALKANES
- General Formula : CnH(2n+1)X ----> X is the halogen
- Electronegativity : Fluorine to Iodine electronegativty decreases ---> increasing atomic radius ---> weaker attraction between nucleus and electrons. Haloalkanes have C-X bond. Bond is polar as ALL halogens are more electronegative than carbon. So X (halogen) with have d- and C will have d+ charge.
- Not soluble in water and C-X bond not polar enough.
- Boiling Point : depends on number of atoms ----> increases with chain length (larger chain - more electrons - stronger van der waals)
- increases down halogen group.
- more branching - lower melting point haloalkanes have higher boiling points than similar alkanes - larger RMM and more polar.
- Reactivity : increases down halogen group - bond enthalpy decreases though bond polarity increases ---> bond enthalpy MORE important than bond polarity.
NUCLEOPHILIC SUBSTITUTION
Nucleophiles : -ve charged ion that attack and form bonds with +ve atoms(electron deficient atoms). Also electron pair donor.
- Hydroxide ion ---> -:OH e.g. haloalkane wih aqueous Na / K hydroxide ---> room temp. ethanol solvent. alcohol and -ve halide ion formed.
- Cyanide ion ---> -:CN nitirle and -ve halide ion produced. Carbon of -CN group is counted in name.
NUCLEOPHILIC SUBSTITUTION : AMMONIA ION
- Ammonia ion ---> :NH3 Ammonia is a nuclophile due to lone pair.
reaction --> excess concentrated solution of ammonia in ethanol - under pressure.
produces primary amine, NH4+ ion and -ve halide ion.
ELIMINATION REACTIONS
Haloalkanes + NaOH / KOH (dissolved in ethanol) ---> Alkene + Hydrogen halide
Z (functional group on same side) and E( funct. group opposite sides) isomers of alkene can also be produced.
OH- ion acts as base - nucleophile - lone pair attacks H+ ion from haloalkane. Conditions : NaOH / KOH with ethanol ---> heated
N.B. Primary haloalkanes tend to react by subst. - tertiary by elimin. Secondary both.
FREE RADICAL SUBSTITUTION - FORM HALOALKANES
Alkane + Halogen (UV light) ---> Haloalkane
- Initiation : X-X bond breaks - absorbs UV light - enough energy to break bond. Breaks homylitically (one electron to each atom) ---> 2 free radicals 2X
- Propagation :
Stage 1 - X takes H atom from alkane ---> HX (stable compound) + alkane free radical
Stage 2 -alkane free radical reacts with X-X ---> haloalkane (stable) + X
chain reaction (Stage 1 repeats)
FREE RADICAL SUBSTITUTION 2
- Termination : free radicals are removed - 3 different ways
1) X + X ----> X2 (halogen molecule)
2) alkane free radical + alkane free radical ----> alkane
3) X + alkane free radical ----> haloalkane
ELECTROPHILIC ADDITION
1) electrophile attracted to C=C bond - takes pair of electrons --> C-C bond formed
2) carbocation (C+) formed
3) -ve ion bonds with carbocation.
-ve ion bons with C with most alkyl groups - most stable.
Reactions : with hydrogen halides e.g. HCl, HBr and HI
ELECTROPHILIC ADDITION 2
Reaction : alkene + halogen ---> dihaloalkane
Test for C=C bond - few drops of bromine solution(brown) to alkene - turns colourless
ELECTROPHILIC ADDITION 3
Reaction : alkene + concentrated sulfuric acid
If water added - alcohol + sulfuric acid formed
Conditions : steam, phosphoric acid catalyst
ALCOHOLS
Primary Alcohols : OH bonded to C atom attached to only 1 other C atom (R group)
Secondary Alcohols : OH bonded to C atom attached to 2 C atoms (2 R groups)
Tertiary Alcohols : OH bonded to C atome attached to 3 C atoms (3 R groups)
Due to hydrogen bonding - high melting + boiling points. Short chains soluble in water.
Making of Ethanol:
ethene + water --> ethanol (phosphric acid catalyst) Hydration
glucose ---> ethanol + carbon dioxide ( anaerobic repiration) Fermentation
ALCOHOLS 2
Combustion : Alcohol + oxygen --> carbon dioxide + water
Oxidation :Primary alcohols
---> aldehyde( RCH=O) + water - alcohol excess, gently heat
dilute acid + potassium dichromate
---> carboxylic acid(RCOH=O) - oxidising agent excess, reflux
concentrated sulfuric acid + excess potassium dichromate( twice as much as used to produce aldehyde)
ALCOHOLS 3
Secondary alcohols ---> ketones (R2C=O) + water (acidified K dichromate)
Ketones do not oxidise further
Tertiary alcohols DO NOT OXIDISE EASILY as they do not have an H bonded to C with hydroxyl (OH) group. Oxidation require C-H bond to break.
ORANGE DICHROMATE IONS TURN GREEN IN OXIDATION
TESTS :
The Tollens' test - solution of silver nitrate and aqueaous ammonia. Oxidises aldehydes but NO AFFECT on KETONES. Deposit of metallic silver formed.
The Fehlings'/ Benedict's test - oxidise aldehydes but NOT KETONES. Blue solution changes to brick red precipitate(copper oxide) - requires heat.
ELIMINATION REACTION 2
DEHYDRATION :
alcohol ---> alkene + water
excess hot concentrated sulfuric/ phosphoric acid
OR aluminimum oxide catalyst
HALOGENS
- ATOM SIZE INCREASES DOWN GROUP
- more shells
- ELECTRONEGATIVITY DECREASES DOWN GROUP
- increased shielding, further away from nucleus.
- MELTING + BOILING POINT INCREASES DOWN GROUP
- larger atoms - more electrons - stronger Van der Waals forces.
- OXIDISING ABILITY DECREASES DOWN GROUP
HALOGENS 2
HALIDE IONS ---> REDUCING ABILITY INCREASES DOWN ION GROUP
ALL CAN DISPLACE A HALOGEN LOWER IN THE GROUP
USES OF CHLORINE :
PURIFY WATER - ALTERNATIVE - solid calcium chlorate - dissolves - chloric acid - reversible reaction
HOUSEHOLD BLEACH
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