Organic chemistry reactions

Alkane to Alkene

cracking - the breaking down of long chained alkanes into shorter chained alkanes and alkenes

catalyst: Al2O3 / SiO2 (zeolite)
conditions: high temperatures (500 c)

• shorter chained alkanes are more useful and more in demand

• produces shorter alkenes for making plastics / polymers

alkane to cycloalkane

reforming - the conversion of straight chained alkanes into branched, cyclic and aromatic hydrocarbons
increases octane rating of fuels (prevent knocking + smoother burning)

C7H16 -> C7H14 + H2
heptane to methylcyclohexane

catalyst: Pt (or Rh) metal on Al2O3 support
conditions: > 500 c

Alkene to Alkane

hydrogenation, H2 (also an addition reaction and reduction - gain of hydrogen)

ethene and hydrogen
C2H4 + H2 -> C2H6

catalyst: Ni
conditions: 150 c
or Pd catalyst at room temp.

does not have the same mechanism as with bromine - H2 is weak electrophile (not very polarisable as small electron cloud)

used commercially to manufacture margarine from hydrogenation of vegetable oils.

Alkene to mono-alcohol

ethene + water (steam) -> ethanol
C2H4 + H2O <-> C2H5OH

conditions: dilute phosphoric acid (H3PO4) as catalyst
300 c, 60 - 70 atm

uses H+ from acid to start the reaction, at the end a H+ is regenerated (catalyst not used up)

Alkene to Diol

oxidation with acidified potassium manganate (VII), KMnO4 (aq)

ethene + potassium manganate (VII) - oxidising agent
C2H4 + H2O + [O] -> CH2(OH)CH2(OH) (ethane-1,2-diol)

reaction in dilute H2SO4

conditions: room temp

observations: purple to colourless

Alkane to Halogenoalkane

Radical substitution in the presence of UV light
causes bromine water to decolourise

initiation: Br2 -> Br• + Br•
propagation: CH4 + Br• -> CH3• + HBr
CH3• + Br2 -> CH3Br + Br•
overall: CH4 + Br2 -> CH3Br + HBr
termination: CH3• + Br• -> CH3Br
CH3• + CH3• -> C2H6
(unwanted side reactions)

Alkene to dihalogenoalkane

electrophilic addition
bromine liquid, Br2 (l)

C2H4 + Br2 -> CH2BrCH2Br
ethene + bromine -> 1,2-dibromoethane

turns from RED to colourless, decolourises

note: bromine water:
C2H4 + Br2 + H2O -> CH2BrCH2OH + HBr
(H2O acts as second nucleophile instead of Br-)

turns from ORANGE to colourless

Alkene to Halogenoalkane

using hydrogen halides (molecule, not ions)

C3H6 + HCl -> CH3CH2CH2Cl (minor product)
or CH3CHClCH3 (major product)

as secondary carbocation more stable than primary
more alkyl groups = greater positive inductive effect = more stable carbocation

alkene to addition polymer

Addition polymerisation - the linking together of small monomer molecules to form a giant polymer molecule

ethene -> polyethene

peroxides as radical initiators under UV light which causes homolytic fission of weak O-O bond

Halogenoalkanes to alcohols

nucleophilic substitution
Dilute NaOH (aq), halogenoalkane dissolved in ethanol
conditions: heat under reflux

CH3Cl + NaOH -> CH3OH + NaCl
reaction mechanism is in one step! (SN2)

this is quicker than with water - OH- is a stronger nucleophile

Halogenoalkanes to alcohols (CP4)

CP4 nucleophilic substitution / hydrolysis
using AgNO3 (aq)
conditions: heat using warm water bath (30-70 c)
compound dissolved in ethanol as dissolves both

CH3CHICH3 + H2O -> CH3CHOHCH3 + HI
water is the nucleophile
produces acidic solution (HI)
Iodide ions formed react with dissolved Ag+ to form a yellow AgI precipitate

investigating the rate of nucleophilic substitution (time taken for precipitate to form using 'x' under flask)
comparing between different halogens in molecule (Cl / Br / I) + classification (1 / 2 / 3)
use water bath to keep temp the same

results: Iodine most reactive > Br > Cl - Iodine has lowest bond enthalpy / weakest = lowest energy needed to break
3 > 2 > 1 - forms tertiary carbocation intermediate (more stable)

Halogenoalkane to nitrile

reaction with potassium cyanide (KCN dissolved in ethanol)!!

conditions: heat under reflux

CH3CH2Br + KCN -> CH3CH2CN + KBr
forms propanenitrile

halogenoalkane to amine

reaction with ammonia (concentrated NH3 solution in ethanol, present in excess)

conditions: heat in a sealed tube (prevent ammonia gas escaping)

CH3CH2Cl + 2NH3 -> CH3CH2NH2 + NH4Cl
forms ethylamine

1st NH3 acts as nucleophile forming new sigma bond to C
2nd NH3 acts as base - accepts a H+ ion to form NH4+

primary amine product could react further with another molecule of chloroethane - N has lone pair (can be nucleophile) so use excess NH3

Halogenoalkane to Alkene

Elimination - generally = reaction in which atom / group of atoms are removed from a molecule to form a new product
- in this context = removal of atoms from adjacent C atoms to form an unsaturated compound

reaction with concentrated ethanolic potassium hydroxide KOH
conditions: heat under reflux

CH3CHBrCH3 + KOH -> CH2=CHCH3 + H2O + KBr
OH- acting as a base

cannot undergo elimination if there is no H atom bonded to an adjacent C atom (to where halogen bonded to)

Alcohol to Chloroalkane (CP6)

Nucleophilic substitution
Substitution - reaction in which a functional group in a compound is replaced with another different functional group

reaction of alcohols with phosphorus (V) chloride PCl5 / concentrated HCl
conditions: anhydrous (react with water when in contact - less toxic HCl formed)

CH3CH2OH + PCl5 -> CH3CH2Cl + HCl + POCl3
1 mol of PCl5 for every -OH

used for test for a covalently bonded -OH group - may react with alkali too
observations: steamy fumes (HCl gas dissolves in water to form droplets of hydrochloric acid) + (vigorous) effervescence
hazard: HCl is an acidic gas + corrosive - use fume cupboard

OR

alcohol + conc HCl
substituting the -OH for a Cl, forming H2O
fun fact: -OH is first protonated to form H2O group which makes a good leaving group

CP6: react (open bung to release pressure form time to time) then add anhydrous calcium chloride (solid and dissolve) to ensure any unreacted OH is in the lower aqueous layer

after in separating funnel, add sodium hydrogencarbonate solution repeatedly to react with any acid / H+ ions
after run off, add anhydrous sodium / magnesium sulphate to dry - appearance becomes less cloudy / goes clear

DISTILL

Alcohol to Bromoalkane

reaction of alcohol with phosphorus (III) bromide PBr3
2P + 3Br2 -> 2PBr3, heat under reflux

3 x CH3CH2OH + PBr3 -> 3 x CH3CH2Br + H3PO3
* not H3PO4

OR

reaction of alcohol with HBr - hydrobromic acid
formed by KBr + conc H2SO4 -> HBr + KHSO4
* not K2SO4

C3H7OH + HBr -> C3H7Br + H2O
swapping -OH for Br, forming H2O

• H2SO4 is added slowly to prevent mixture boiling over as reaction is exothermic

• immerse in beaker of cold water as product has low boiling point

• (heat under reflux) and DISTILL

• purifying: discard aqueous layer, (conc hydrochloric acid to remove unreacted alcohol), sodium hydrogencarbonate to remove remaining acid, shake and open tap to release CO2 gas, anhydrous salt, decant and DISTILL again

Alcohol to Iodoalkane

reaction of alcohol + PI3 phosphorus (III) iodide made in situ (gear under reflux?)
formed by 2P + 3I2 -> 2PI3 from iodine and moist red phosphorus

3 x CH3CH2OH + PI3 -> 3 x CH3CH2I + H3PO3

not reaction with HI - I- ions reduce H2SO4 (unwanted side reaction), instead of substitution

Alcohol to Alkene

Dehydration/Elimination

conditions: heat under reflux
concentrated phosphoric acid (H3PO4) as a catalyst
(dilute is for the reverse reaction)

CH3CH2CH2OH -> CH3CH=CH2 + H2O

need H atom attached to an adjacent C
use H3O+ from catalyst to start the reaction - H+ added to alcohol
remaining H2O takes a H from the adjacent C atom to regenerate catalyst

Primary alcohol to carboxylic acid (CP5)

Oxidation: acidified potassium dichromate as oxidising agent
K2CrO7 + H2SO4
first oxidised to an aldehyde (-CHO) then COOH
conditions: heat under reflux (and also distil after)
to make aldehyde only = heat AND distil

CH3CH2CH2OH -> CH3CH2CHO + 2H+ + 2e-
CH3CH2CHO + H2O -> CH3CH2COOH + 2H+ + 2e-

CH3CH2CH2OH + 2[O] -> CH3CH2COOH + H2O

observations: turns from orange to green (Cr3+)

*add ethanol drop by drop to dichromate / acid in ice water bath, remove from ice water bath, heat under reflux (but through water bath), heat and distil

secondary alcohols to ketones

Oxidation: acidified potassium dichromate as oxidising agent
K2CrO7 + H2SO4
are not further oxidised

CH3CHOHCH3 -> CH3COCH3 + 2H+ + 2e-
CH3CHOHCH3 + [O] -> CH3COCH3 + H2O

observations: turns from orange to green

heat under reflux

The continuous cycle of boiling and condensing
allowing increased rate of reaction + complete reaction
allowing reactants to stay in contact with each other for a long period of time
preventing loss of volatile liquids from a heated reaction vessel

reactions: halogenoalkane to alcohols, nitriles and alkenes
alcohol to alkenes and carboxylic acids

only heat: halogenoalkanes to amines (sealed tube), CP4 halogenoalkane to alcohol (AgNO3)

alcohol to aldehydes, alcohol to chloroalkanes and bromoalkanes: heat and distil

tertiary alcohols

not further oxidised as there is no H atom attached to the C atom the -OH is attached to
C-H needs to break for C to form C=O

Distillation

- round bottom flask + anti-bumping granules, heat, thermometer opposite condenser opening
- downward sloping condenser and water in from the bottom
- no gaps on LHS, open collecting vessel

Anti-bumping granules

allow smooth and even boiling / prevent violent boiling by providing sites / surfaces for (smaller) bubbles to form

reactions in ethanol

halogenoalkanes to alcohols, amines, nitriles and alkenes (everything with halogenoalkanes)

to dissolve halogenoalkane so that reactants are miscible