Edexcel Chemistry - Topic 17: Organic II
- Created by: Ryan C-S
- Created on: 09-04-18 19:25
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 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)
Racemic Mixtures
- 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
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
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
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 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
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 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
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 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
- 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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