Optical isomerism Flashcards
When does optical isomerism occur?
when a carbon atom has 4 different atoms or groups of atoms attached to it, resulting in an asymmetrical carbon/ chiral centre
What are enantiomers?
2 molecules that are non-superimposable isomers that are mirror images of each other
What is the difference between enantiomers?
physical properties the same- but when exposed to plane polarised light, the enantiomers rotate the plane in opposite directions the same amount
Which enantiomer rotates the plane CLOCKWISE?
dextrorotary
Which enantiomer rotates the plane ANTICLOCKWISE?
laevorotary
How is rotation of light measured?
using a polarimeter
What is a 50:50 mixture of equal amounts of enantiomer called?
a racemic mixture/ a racemate
Why is a racemic mixture optically inactive?
contains equal amounts of both enantiomers , so is optically inactive because the optical effects of each enantiomer cancel each other out
What happens when optically active molecules are made in nature?
only one enantiomer is formed
Why is only one enantiomer formed in nature?
enzymes are stereospecific, as they themselves are optically active, so only react either one enantiomer and produce one enantiomer
What are the two ways aldehydes and ketones undergo nucleophilic addition?
a) hydride ion (:H-) from NaBH4 in reduction in aqueous solution
b) the cyanide ion (:CN-) provided by KCN, followed by dilute acid
What is the name of NaBH4?
sodium tetrahydridoborate (III)
What are the products or nucleophilic additon of NaBH4 and aldehydes/ketones
aldehyde- primary alcohol
ketone- secondary alcohol
What happens between aldehydes/ketones and KCN?
undergo nucleophilic addition with dilute acid to form nitriles
When are chiral centres formed from ketones?
when the starting ketone is asymmetrical
Why is potassium cyanide dangerous?
highly toxic, HCN is also extremely toxic, also highly flammable
Why is the -CN useful?
increases length of carbon chain, easily converted into other functional groups once added
What does the reaction of aldehydes/ketones with acidified KCN produce?
a racemic mixture because it has a chiral centre
What is the science behind a racemic mixture?
because the carbonyl group is planar, so susceptible to attack by the :CN- nucleophile is equally likely from either side of the plane
What is a non-specific way of reducing aldehydes/ketones?
reduced with hydrogen in the presence of a nickel catalyst (catalytic hydrogenation) at 150°C
Why is it non-specific?
molecules containing both C=O and C=C will become completely desaturated
Why is the reduction by NaBH4 specific?
nucleophiles cannot attack alkenes (electron rich double bond repels nucleophiles), so they can target aldehydes/ketones
Why are carboxylic aids soluble up to bromine (included)?
can form hydrogen bonds with water
Why do carboxylic acids have a higher boiling point than comparable alkenes?
they can form hydrogen bonds with each other
What do carboxylic acids react with?
metals, bases, metal carbonates/hydrogenocarbonates
What is the equation for the reaction between carboxylic acids and metals?
Give the example of Mg and ethanoic acid
metal + acid -> salt + hydrogen
magnesium + ethanoic acid -> magnesium ethanoate + hydrogen
Mg + 2CH3COOH -> (CH3COO-)2 Mg2+ + H2
What is the equation for the reaction between carboxylic acids and bases?
Give the example of sodium hydroxide and ethanoic acid
base + acid -> salt + water
sodium hydroxide + ethanoic acid -> sodium ethanoate + water
NaOH + CH3COOH -> CH3COO- Na+ + H2O
What is the equation for the reaction and difference between carboxylic acids and metal carbonates/hydrogencarbonates?
Give the example of calcium carbonate and propanoic acid
metal carbonate + acid -> salt + water + carbon dioxide
calcium carbonate + propanoic acid -> calcium propanoate + water + carbon dioxide
CaCO3 + 2CH3CH2COOH -> (CH3CH2COO-)2 Ca2+ + H2O + CO2
difference is that for carbonates there is 2 moles of acid but for hydrogencarbonates there is only 1 mole
How do you test for carboxylic acids?
-add sodium hydrogencarbonate
-bubbles of CO2 present if it is a carboxylic acid
How do you make an ester through esterification?
carboxylic acid + alcohol ⇌ ester + water
What is the condition for esterification?
concentrated H2SO4 acid catalyst
Why is esterification not ideal for making esters?
it is a reversible reaction so has a low yield
How do you name an ester?
first part is from the alcohol- change the name to -yl
methanol -> methyl
second part is from the carboxylic acid- change the name to -oate
ethanoic acid -> ethanoate
What are esters used for?
perfumes, artificial food flavourings, plasticisers, solvents
Esters and carboxylic acids are…?
functional group isomers
What is the backward reaction of esterification?
ester hydrolysis- can happen in either acidic or alkaline solution
Describe ester hydrolysis in acidic solution?
backward reaction, ester + water ⇌ carboxylic acid + alcohol, with a concentrated sulphuric acid catalyst
Describe ester hydrolysis in alkaline solution?
boil with NaOH, quicker and leads to a greater yield due to a completed reaction
ester + sodium hydroxide –> sodium salt of carboxylic acid + alcohol
What happens when you boil fats in NaOH?
makes soap
Why does glycerol have a high viscosity?
forms hydrogen bonds, can also stop things drying out due to its ability to attract water
What is the IUPAC name for glycerol?
propane-1,2,3-triol
What are acyl chlorides and acid anhydrides?
derivatives of carboxylic acids
How are acyl chlorides named?
after the carboxylic acid, ie- ethanoic acid becomes ethanoyl chloride
Why are acyl chlorides and acid anhydrides so reactive?
both the O and the Cl are more electronegative than the carbon, so the carbon becomes electron deficient and readily attacked by nucleophiles
How are acid anhydrides named?
after the carboxylic acid, ie- ethanoic acid becomes ethanoic anhydride
What is acylation?
adding an acyl group to a molecule (R-C=O)
Why are acyl chlorides and acid anhydrides more reactive than their corresponding carboxylic acid?
because they have electron deficient carbon atoms that are susceptible to nucleophilic attack
What are the four nucleophilic addition-elimination reactions and their products?
1) water –> carboxylic acids + HCl
2) alcohols–> esters + HCl
3) 2ammonia–> amides + ammonium chloride
4) 2primary amines–> N-substituted amides + alcohol-ammonium chloride
How do you test for acyl chlorides vs acid anhydrides?
-add water
-measure pH with pH meter
-pH is around 3 for acid anhydride as the carboxylic acid produced is a weak acid
-pH is around 0/1 for acyl chloride as when it reacts with water, HCl is also made which is a strong acid, in addition to the carboxylic acid
Why use an acid anhydride over an acyl chloride?
-cheaper
-less corrosive
-less reactive and less susceptible to hydrolysis
What happens in the nitration of benzene an its conditions?
step 1-making the electrophile, (+NO2)
step 2- electrophilic substitution
conditions- concentrated H2SO4 catalyst, 50C, as any higher than 50C multiple substitutions can occur
What is the equation for making the nitronium ion?
HNO3+2H2SO4 –> +NO2 + H3O+ + 2HSO4-
What can nitrated benzene derivatives be used for?
explosives
How do you make soap?
triglycerides + sodium hydroxide –> (boil) SOAP + glycerol
What are common uses of esters?
-perfumes
-plasticisers
-solvents
What is one similarity and one difference between the acyl chlorides and acid anhydrides in acylation reactions?
both can provide the acyl group for the acylation of another molecule, but the reaction of the acid anhydrides are less vigorous
Why are acid anhydride and acyl chlorides more reactive than carboxylic acids?
C-Cl and C=O bonds are more polar as Cl and O are more electronegative than carbon, so carbon atom is more reactive as it is more electron deficient
