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Edexcel Chemistry – Topic 17: Organic II

Chirality

  • 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 superimposablewith 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)

Racemic Mixtures

  • 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

Aldehydes: Properties

  • 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

Ketones: Properties

  • 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

Image result for ketones

Oxidation of Aldehydes

  • 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

Oxidation of Ketones

  • 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

Reduction of Carbonyls

  • 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

Formation of Hydroxynitriles

  • 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

Reaction of Carbonyls with 2,4-DNP

  • 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 of Carbonyls with Iodine

  • 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

Carboxylic Acids: Properties

  • 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

Strength of Carboxylic Acids

  • 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

Preparation of Carboxylic Acids

  • 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

Image result for alcohol to carboxylic acid

Reduction of Carboxylic Acids

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

LiAlH4 acts as a reducing agent

Hydrolysis of Nitriles

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

Salt Formations of Carboxylic Acids

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 with Phosphorus(V) Chloride

  • 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

Oxidation of Methanoic Acid

  • 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

 

Acyl Chlorides: Properties

  • 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 of Acyl Chlorides with Water

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

Reaction of Acyl Chlorides with Ammonia

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

Reaction of Acyl Chlorides with Primary Amides

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

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

Esters: Properties

  • 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 chloridereacting 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 smellingvolatile and don’t react with water

Esterification

  • 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 reactionslow 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

Hydrolysis of Esters

  • 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

  • Triglycerides are naturally occuring esters consisting of three carboxylic acidsbonded 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 watermolecule
  • 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

  • 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

Terylene

  • 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

Poly(lactic acid)

  • 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

Chemical Reactivity of Poly(esters)

  • 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

 

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