Edexcel Chemistry - Topic 17: Organic II

• Optical Isomerism (Chirality) occurs in carbon compounds containing 4 different groups attached to a carbon e.g. Amino Acids.
• Chiral molecules are mirror images of one another and are not superimposable with one another.
• Two molecules that are optical isomers of each other are enantiomers.
• Chiral molecules have similar physical and chemical properties but rotate plane polarised light in different directions
• The Dextrorotatory enantiomer will rotate light clockwise (+)
• The Laevorotatory enantiomer will rotate light anticlockwise (-)
• Some enantiomers give a different flavour (e.g. spearmint and caraway) and some drugs have one safe enantiomer (e.g. Thalidomide)
  

• A racemic mixture contains a 50:50 mixture of two enantiomers
• It will not rotate plane-polarised light
• Racemic mixtures form when a trigonal planar reactant or intermediate is attacked from both sides during a reaction mechanism (e.g. HCN reacting with aldehydes and asymmetrical ketones)
• There is an equal chance of forming each enantiomer so a racemic mixture forms

• General formula: RCHO
• Suffix = -al (e.g. Ethanal)
• Contains a carbonyl group (C=O bond)
• Can form permanent dipole interactions and are soluble in water
• Aldehydes are used in preservatives, flavourings and perfumes

• General Formula: R1COR2
• Suffix = -one (e.g. Propanone)
• Contains a carbonyl group (C=O bond)
• Can form permanent dipole interactions and are soluble in water
• Ketones are used in nail polish removers, embalming fluids, perfumes and pesticides

• Reaction: Aldehyde --> Carboxylic Acid
• Reagent: Potassium Dichromate(VI) and dilute sulphuric acid
• Conditions: Heat under reflux
• Observations: Colour change - Orange > Green

• Aldhydes can also be oxidised to Carboxylic Acids by addition of either Tollens Reagent or Fehling's Solution
• Tollens Reagent - Formed by mixing aqueous ammonia with silver nitrate to form [Ag(NH3)2]+
  Conditions: Heat gently
  Reaction: Aldehydes are oxidised into a carboxylic acid and the silver(I) ions reduced to silver atoms
  Observations: A silver mirror forms

• Fehling's Solution - A solution of Cu2+ ions
  Conditions: Heat gently
  Reaction: Aldehydes are oxidised into a carboxylic acid and the Cu2+ ions reduced to Cu2O
  Observations: Colour Change - Blue Solution > Red Precipitate

• Ketones DO NOT oxidise on reaction with Potassium Dichromate(VI) and dilute sulphuric acid and the orange colour will remain
• Addition of Fehlings solution causes the blue colour of the copper(II) ions to remain
• Addition of Tollen's Reagent does not cause the silver ions to be reduced to silver atoms
• Ketones are unable to be oxidised because unlike an aldehyde where there is a free hydrogen which can be oxidised further, a ketone has two carbon chains next to the carbonyl group

• Reaction: Aldehyde/Ketone --> Alcohol
• Reagents: Lithium tetrahydridoaluminate (LiAlH4)
• Conditions: Room temperature and pressure

LiAlH4 acts as a reducing agent. Other reducing agents (e.g. Sodium tetrahydridoborate - NaBH4) can be used

Aldehydes are reduced to a Primary Alcohol
Ketones are reduced to a Secondary Alcohol

• Reaction: Aldehyde/Ketone --> Hydroxynitrile
• Reagents: HCN in the presence of KCN
• Conditions: Room temperature and pressure
• Mechanism: Nucleophilic Addition

When naming a nitrile, the CN becomes part of the main chain

CH3COCH3 + HCN --> CH3C(OH)(CN)CH3
propanone + hydrogen cyanide --> 2-hydroxy-2-methylpropanenitrile

• 2,4-dinitrophenylhydrazine (2,4-DNP) reacts with both Aldehydes and Ketones.
• The product is an orange precipitate so the reaction can be used as a test for a carbonyl group in a compound
• Fehling's Solution or Tollen's Reagent have to be used to distinguish between whether a compound is an Aldehyde and a Ketone

• Reaction: Carbonyl --> Triiodomethane
• Reagents: Iodine and NaOH
• Conditions: Warm very gently
• Observations: Yellow crystaline precipitate with an antiseptic smell

The reaction only works if there is a methyl group next to the C=O bond. Ethanal is the only aldehyde that reacts. More commonly are methyl ketones e.g. Propanone.
The reaction is called the Iodoform Test

CH3COCH3 + 3 I2 + 4 NaOH --> CHI3 + CH3COONa + 3NaI + 3H2O

• General Formula: RCOOH
• Suffix = -oic acid
• Carboxylic Acids can form hydrogen bonds so are soluble in water
• Carboxylic Acids are weak acids in water and only slightly dissociate, but are strong enough to displace carbon dioxide from carbonates
• Carboxylic Acids delocalise (the pi charge cloud spreads out) to form stable ions/salts
• Carboxylic Acids are found in vinegar and cream of tartar

• Longer carbon chains pushes electron density onto the COO- ion making it more negative and less stable - therefore the acid is weaker
• Propanoic acid is less acidic than ethanoic acid
• Highly electronegative chlorine atoms withdraw electron density from the COO- ion making it less negative and more stable - therefore the acid is stronger

• Reaction: Primary Alcohol/Aldehyde --> Carboxylic Acid
• Reagent: Potassium Dichromate(VI) and dilute sulphuric acid
• Conditions: Use of excess dichromate and heat under reflux
• Observation: Colour Change - Orange > Green

• Reaction: Carboxylic Acid --> Primary Alcohol
• Reagents: Lithium tetrahydridoaluminate (LiAlH4) in dry ether
• Conditions: Room temperature and pressure

LiAlH4 acts as a reducing agent

• Reaction: Nitrile --> Carboxylic Acid
• Reagents: Dilute hydrochloric/sulphuric acid
• Conditions: Heat under reflux

ACID + METAL (Na) --> SALT + HYDROGEN
2CH3COOH + 2Na --> 2CH3COO-Na+ + H2

ACID + ALKALI (NaOH) --> SALT + WATER
CH3COOH + NaOH --> CH2COO-Na+ + H2O

ACID + CARBONATE (Na2CO3) --> SALT + WATER + CARBON DIOXIDE
2CH3COOH + Na2CO3 --> 2CH3COO-Na+ + H2O + CO2

*Effevescence from the production of CO2 when a carboxylic acid reacts with Sodium Carbonate can be used as the test for a Carboxylic Acid

• Reaction: Carboxylic Acid --> Acyl Chloride
• Reagents: Phosphorus(V) Chloride - (PCl5)
• Conditions: Room temperature and pressure
• Observations: Steamy fumes of HCl will form

CH3COOH + PCl5 --> CH3COCl + POCl3 + HCl

• Carboxylic acids cannot be oxidised with the exception of methanoic acid as it has a structure similar to an aldehyde
• Methanoic acid is oxidised into carbonic acid (H2CO3)

HCOOH + [O] --> HOCOOH

• General Formula: RCOCl
• Suffix = -yl Chloride
• Acyl Chlorides are a carboxylic acid derivative that are more reactive
• The chlorine is more easily lost because of less effective delocalisation making Acyl Chlorides more reactive
• Acyl Chlorides are used for creating other organic chemicals due to their reactivity

• Reaction: Acyl Chloride --> Carboxylic Acid
• Reagent: Water
• Conditions: Room temperature and pressure
• Observations: Mitsy fumes of HCl

• Reaction: Acyl Chloride --> Primary Amide
• Reagent: Ammonia
• Conditions: Room temperature and pressure
• Observations: White smoke of NH4Cl produced

• Reaction: Acyl Chloride --> Secondary Amide
• Reagents: Primary Amine
• Conditions: Room temperature and pressure

RCOCl + 2 CH3NH2 --> RCONHCH3 + CH3NH3+Cl-

• General Formula: R1COOR2
• Suffix = -yl -oate
• Esters form on the reaction of an alcohol with either a carboxylic acid or an acyl chloride
• The -yl part of the name comes from the alcohol reacting e.g. Methanol = Methyl-
• The -oate part of the name comes from the carboxylic acid or acyl chloride reacting e.g. Ethanoic Acid = Ethanoate
• Esters don't form hydrogen bonds and are almost insoluble in water
• Esters are used in perfumes as they are sweet smelling, volatile and don't react with water

• Reaction: Carboxylic Acid + Alcohol ⇌ Ester + Water
• Catalyst: Sulphuric acid
• Conditions: Heat under reflux

The formation of an ester with a carboxylic acid is a reversible reaction, slow and a low yield is produced

• Reaction: Acyl Chloride + Alcohol --> Ester + HCl
• Conditions: Room temperature and pressure
• Observations: Steamy fumes of HCl are evolved

The formation of an ester from an acyl chloride is a better reaction because:

• It is quicker
• It is not a reversible reaction
• It produces higher yields and is more efficient

• Reaction: Ester + Water ⇌ Alcohol + Carboxylic Acid
• Reagents: Dilute hydrochloric acid and excess water
• Conditions: Heat under reflux

This reaction is reversible and doesn't give a good yield of either product

• Reaction: Ester --> Carboxylic Acid Salt + Alcohol
• Reagents: Sodium Hydroxide
• Conditions: Heat under reflux

The anion is resistant to attacks by weak nucleophiles e.g. alcohols so the reaction is not reversible

• Triglycerides are naturally occuring esters consisting of three carboxylic acids bonded to propane-1,2,3-triol (glycerol) important for energy stores in biological systems
• Ester bonds are links between the carboxylic acids and glycerol that form in a condensation reaction 
• Condensation reactions are reactions that join two molecules by releasing a water molecule
• The carboxylic acid chain can be either saturated (only C-C bonds) or unsaturated (contains C=C double bonds)
• Saturated fats are solids at room temperature as the chains can pack together easily so the Van der Waals forces act stronger increasing melting point

• Polyesters are a condensation polymer
• Condensation polymers add two monomers together by releasing a water molecule
• Polyesters can be formed by the following reactions:
  Dicarboxylic Acid + Diol --> Poly(ester) + Water
  Diacyl Chloride + Diol --> Poly(ester) + HCl
• Using carboxylic acids to make a polyester would mean an acid catalyst is required and an equilibrium will be established
• Using acyl chlorides to make a polyester results in a more reactive reaction that goes to completion without the need for a catalyst but does result in the production of toxic HCl fumes

• Monomers: Benzene-1,4-dicarboxylic acid and Ethane-1,2-diol
• Terylene is used in clothing because it is strong, flexible, hard waring and washable
• Terylene can be treated by stretching and heating to make it stronger for use in drinks bottles and food containers

• Monomer: 2-hydroxypropanoic acid
• Poly(lactic acid) - PLA - is a biodegradable polymer used in plastics, plannt pots, disposable nappies and absorbable surgical sutures (stiches)
• The reaction joins muliple monomers of lactic acid together in a condensation polymer

• Polyesters are biodegradable (can be broken down by hydrolysis)
• Polyesters can be hydrolysed by acids and alkalis

With HCl

• A polyester splits up into the original dicarboxylic acid and diol

With NaOH

• A polyester splits up into the a diol and a dicarboxylic acid salt