AS AQA Chemistry Unit 2 Organics

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  • 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.
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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.
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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.

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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.

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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)

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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

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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

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ELECTROPHILIC ADDITION 2

Reaction : alkene + halogen ---> dihaloalkane

Test for C=C bond - few drops of bromine solution(brown) to alkene - turns colourless

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ELECTROPHILIC ADDITION 3

Reaction : alkene + concentrated sulfuric acid

If water added - alcohol + sulfuric acid formed 

Conditions : steam, phosphoric acid catalyst

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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




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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)

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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.

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ELIMINATION REACTION 2

DEHYDRATION :

alcohol ---> alkene + water

excess hot concentrated sulfuric/ phosphoric acid

OR aluminimum oxide catalyst

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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
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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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