Unit 4- Further organic chemistry Flashcards
What are the two forms of stereoisomerism?
geometric and optical isomers
what are geometric isomers?
isomers which have the same gruop of atoms but in a different spatial arrangement
How does geometric isomer occur?
due to the lack of rotation by the carbon carbon double bond
What is chirality?
when two objects are non superimposable and are mirror images of eachother, they are chiral. This feature is exclusive to asymmetrical objects
How is a molecule chiral?
if there are four different groups attached to a central carbon where the molecules are non superimposable and are mirror images
If the isomers are mirror images of eachother, what are they reffered to as?
enantiomers
How to tell between two enantiomers?
different effect on plane of polarised light where one enantiomer rotates the plane of light clockwise while the other anticlockwise
What type of wave is the plane of polarised light?
transverse
When an enantiomer is placed between two polaroids, what happens?
the second polaroid must be rotated to allow the light to pass through, this is the angle of rotation
What mixtures have no effect on the plane polarised light?
racemic mixtures
why do racemic mixtures have no effect on plane polarised light?
they are formed in a 1:1 ratio therefore the clockwise rotation is cancelled by the anticlockwise rotation
What is the functional group of an aldehyde?
a bonded hydrogen atom and a double bonded oxygen atom to a carbon atom
H-C=O
What is the functional group of a ketone?
A double bonded oxygen atom to a carbon
-C=O
what is the C=O functional group referred to as?
a carbonyl group
what are the properties of the carbonyl group?
- greater boiling and melting temperature in comparison to alkanes but lower than alcohol
- straight chain isomers are more packed therefore stronger than branched chain
- aldehydes are slightly more volatile than ketones
- aldehydes and ketones are miscible with water but this decreases as chain length increases
Why are aldehydes and ketones soluble in water?
Due to the ability to hydrogen bond with water
What are the effects of a carbonyl group?
- pi and sigma bonds present
- the bond is permanently polarised
- cannot hydrogen bond between themselves
- permament dipole-dipole interactions
Describe the oxidation reaction of aldehydes and ketones
- aldehydes oxidise due to the hydrogen atom
- oxidises to carboxylic acid
- warming with potassium dichromate (VI) acidified with sulfuric acid
- ketone does not readily oxidise
Describe the reaction with fehlings and benedicts
- If an aldehyde is warmed, it is oxidised to a carboxylic acid
- This reduces the fehlings giving a red-brown precipirate
- precipitate is of copper(I) oxide
- ketone does not react
How is tollens reagent formed?
- produced by dissolving nitrate in water and adding aqeuous ammonia
- silver (I) oxide is formed but dissolves as more ammonia is added
- forms the complex diammine silver (I) ion
Describe the reaction with tollens reagent
- Tollen is warmed gently with aldehyde
- the aldehyde is oxidised
- silver (I) ions are reduced forming silver metal
- produces a silver mirror
- ketone does not react
What are the oxidation reactions of aldehydes and ketones?
- oxidation by potassium dichromate (VI)
- With fehlings and benedicts
- With tollens reagent
What is the reduction reaction of aldehydes and ketones?
- aldehydes can be redcued to primary alcohols
- ketones can be reduced to secondary alcohols
- Lithium tetrahydridoalumate (LiAlH4) dissolved in dry ether can be used
- sodium tetrahydridoborate (NaBH4) dissolved in ethanol can be used
What is the addition reaction of aldehydes and ketones?
- addition reaction with hydrogen cyanide (HCN)
- produces hydroxynitriles
- occurs in presence of cold alkali and potassium cyanide
- the equilibrium K+ + CN- + H2O à K+ + HCN + OH- is formed
What are the steps of the addition of hydrogen nitrile to an aldehyde or ketone?
- Nucleophile CN- attacks the central carbon displacing a pair of electrons in the double bond and pushing the elctrons to the oxygen atom
- negatively charged oxygen acts as a base and gains a proton
What is the reaction between DNPH and aldehydes and carboxylic acids?
- Reacts with bradys reagent to form a yellow/orange/red solid
- DNPH (Bradys) is oringally a red-orange solid
- condensation reaction
- forms DNPH derivatives
How are the DNPH derivatives used to identify a ketone or aldehydes?
different boiling and melting temerature specific to the aldehyde or ketone it is formed for
Why is DNPH used?
DNPH derivatives are less soluble therefore crystallise easier
Describe the reaction for iodine in the presence of alkali with aldehydes and ketones
- aldehyde is warmed with iodine in alkaline
- aldehyde reacts with iodine to form triodoethanal
- triodoethanal reacts with alkali to form triodomethane
- triodometahne (iodoform) is an insolbule yellow solid with a characteristic smell
What is the functional group of a carboxylic acid?
-COOH where there is a OH bonded and a double bonded oxygen to a carbon
What is a COOH group called?
carboxyl group
What are the two factors which affect the properties of carboxylic acids?
polarity of the carboxyl group and the length of the carbon chain
As the carbon chain length increases in carboxylic acid increases, what happens to the caroxyl group?
the effect is decreased
What are the properties of carboxylic acids?
- due to two polarised groups, relatively strong intermolecular forces are present leading to a high boiling and melting temperature
- can exist as hydrogen bonded dimers thus increasing the boiling and melting temperature
- small chain carboxylic acids soluble in water due to hydrogen bonding
What are the two methods to produce carboxylic acids?
- oxidation
- hydrolysis of a nitrile
How to produce a carboxylic acid by hydrolysis?
- boil with dilute acid
- CH3CH2CN + 2H2O + H+ –> CH3CH2COOH + NH4+
What are the reactions carboxylic acids can undergo?
- reduction
- neutralisation
- with phosphorus pentachloride
- with alcohol
Describe the reduction of carboxylic acids
- can be reduced to a primary alcohol only
- LiAlH4 in dry ether can be used
- addition of water destroys any excess LiAlH4
- water produced
Describe the neutralisation of carboxylic acids
- Reacts with a base
- Forms a salt and water
- salt formed is ionic and soluble
- e.g. CH3COOH + NaOH –> CH3COO- Na+ + H2O
How can carboxylic acids undergo neutralisation reactions?
- carboyxlic acids dissociates in water
- forms carboxylate ions and hydrogen ions
- acts as an acid
Describe the reaction with phosphorus pentachloride
- reacts with solid phosphorus pentachloride (PCl5)
- Forms an acyl chloride
- Forms white fumes of hdyrogen chloride
- Can be used to test for -OH
- e.g. CH3COOH + PCl5 –> CH3COCl + POCl3 + HCl
What is formed in the reaction between carboxylic acids and alcohols?
Esters (esterification)
Describe the reaction between a carboxylic acid and water
- presence of a strong acid catalyst
- forms water as a product
- can also be prepared using an acyl chloride and acid anhydride
What are the two derivatives of carboxylic acids?
- acyl chlorides
- esters
what is the functional group for an ester?
a C-O-C bond joining the alcohol and carboxylic acid
What is the functional group for an acyl chloride?
A double bonded oxygen and a chlorine atom bonded to a carbon
What are the reactions acyl chlorides undergo?
- With water
- With alcohol
- With concentrated ammonia
- With amines
Describe the reaction between an acyl chloride and water
- hydrolysed rapidly in cold water
- hydrogen chloride fumes released
- forms a carboxylic acid
Describe the reaction between an acyl chloride and an alcohol
- Forms an ester
- takes place upon mixing without heating
- forms hydrogen chloride
Describe the reaction between acyl chlorides and concentrated ammonia
- produces amides
- upon mixing without heating
- produces HCl where this reacts further
- HCl + NH3 –> NH4Cl
describe the reaction between acyl chlorides and amines?
forms a substituted amide
What is a hydrolysis reaction regarding an ester?
when the ester splits up into a carboxylic acid and alcohol with water being used
Describe the base hdyrolysis of esters
- hydrolysed by boiling with potassium hydroxide solution
- can be called saponification
- when the acid is released, neutralised by the alkali to form a potassium salt
- Ethyl ethanoate + OH- à ethanoate ion + ethanol
Describe the acid hydrolysis of esters
- Boiled with water and a dilute acid is used
- provdes hydrogen ions which catalyses the reaction
- Ethyl ethanoate + water à ethanoic acid + ethanol
What is a condensation polymer?
when dicarboxylic acids and a diol react forming a polymer with water also being formed
what can the polymers be used for?
plastic bottle drinks
