topic 17 Flashcards
what is optical isomerism a result of
optical isomerism is a result of chirality in molecules with a single chiral centre
what does it mean if a molecule is chiral
if two objects/ molecules are mirror images of each other and they are not superimposable then they are said to be chiral
what does non-super imposable mean
non super imposable means that two images/molecules with the same structure (but one is mirrored) dont match when you put them on top of each other
how does chirality occur
- chirality is a property exclusive to asymmetrical molecules with different groups attached to a central carbon atom
- has no line of symmetry
how can you identify a chiral centre
carbon centre with 4 different groups attached
how can you indicate whether a molecule has a chiral centre
using an *
what does it mean if a molecule has a chiral centre
two possible isomers that are mirror images of each other → optical isomers
he mirror images will be non super imposable to each other
what is the effect of different arrangements of molecules
- the orientation can lead to dramatically different effects on biological organisms which is why we want to separate them
- example of thalidomide
what are enantiomers
each of a pair of molecules that are mirror images of each other
the different isomers are called enantiomers
enantiomers are non-superimposable
what is plane polarised light
light in one plane → travel in one direction
what is a racemic mixture
enantiomers are made in a 50/50 mix which is known as a racemic mixture
what is the effect of individual enantiomers on plane polarised light
individual enantiomers can cause plane polarised light to rotate either to the right or to the left
If single enantiomer is produced
what is the effect of a racemic mixture on plane polarised light and why
plane polarised light has no effect on a racemic mixture because there is an equal mix of left and right enantiomers
what is the SN1 mechanism
- takes place in two steps
- R- represents alkyl groups
- central carbon has 4 different groups so has a chiral centre
- needs to have a chiral centre or it wont work
recall/draw the steps of the SN1 mechanism with the example of 2-bromo,2-methylpropane
what happens if the intermediate is planar in the SN1 mechanism
if the intermediate (carbocation has lost a bond) is planar, the nucleophile can either attack from above or below
- if the attack happens from above it will be a different shape compared to if the attack happens from below
- different arrangement depending on where the nucleophile has attacked from
what happens if bonds are added/lost from the molecule
when bonds are added/lost the shape changes (show this when drawing mechanisms)
what is the product of the SN1 mechanism
racemic mixure
forms enantiomers which means they can rotate plane polarised light which is known as optical activity
what is the evidence for the SN1 mechanism
the product of the SN1 mechanism wont have an effect on plane polarised light
what is a transition state
one group joins as another group comes off
draw the SN2 mechanism with the example of 2-bromo,2-methylpropane
occurs in one step
what is the product of the SN2 mechanism
one product
doesnt have a racemic mixture
product will rotate plane polarised light
which mechanism does a tertiary carbocation undergo
tertiary carbocation should undergo SN1 mechanism
tertiary carbocation is most stable so can only exist as a carbocation for a short period of time
which mechanism des primary carbocations undergo
so for SN2 can only occur with a primary or secondary carbocation as they are less stable
what is the functional group for carbonyl compounds
- carbonyl compounds → contain carbonyl group C double bond O
- gives them the functional group -CO
what are the most common carbonyl compounds
aldehydes and ketones
what does the -CO functional group allow carbonyl compounds to do
allows these molecules to form hydrogen bonds with water
this means that aldehydes and ketones are soluble in water as they can form hydrogen bonds with water molecules
how do hydrogen bonds form between carbonyl compounds and water
a hydrogen bond forms between a lone pair of electrons on the oxygen atom on the C=O functional group and a positive dipole region on a hydrogen atom
why cant carbonyl compounds form hydrogen bonds between other carbonyl compounds
aldehydes and ketones dont have a slightly positive hydrogen atom, therefore they dont form hydrogen bonds between molecules
what forces exist between carbonyl compounds
only london forces are present between these molecules
how can aldehydes be produced
they are produced from the initial oxidation and distillation of primary alcohols
what is the structure of aldehydes
- They have a carbonyl group on a carbon atom at the end of the chain
- this carbon is only attached to one other carbon atom
- this gives them the functional group -CHO
draw an example of the functional group of aldehydes
how can carboxylic acid be produced
aldehydes can be oxidised further in the presence of acidified potassium dichromate (VI) to produce carboxylic acids
what is the structure of ketones
have a carbonyl group on a carbon that is attached to two other carbon atoms
-C=O functional group
how can ketones be produced
produced from the oxidation of secondary alcohols with acidified potassium dichromate (VI)
draw an example of a ketone
write the equation for the oxidation of ethanol to form an aldehyde using displayed formula
write the equation of ethanal (Aldehyde) being further oxidised under reflux to form a carboxylic acid
write the equation for the oxidation of a secondary alcohol (propan-2-ol) to a ketone
what can potassium dichromate (VI) be used for
used as an oxidising agent for oxidation of alcohols
how do you know when an alcohol has undergone oxidation
- potassium dichromate (VI) is reduced when alcohol is oxidised
- results in a colour change from orange to green
what can be used to test for aldehydes
tested using tollens reagent or fehlings solution
how can fehlings solution be used to test for aldehydes
- fehlings/tollens acts as an oxidising agent
- a few drops of fehlings is added and the test tube is gently warmed
- if aldehyde present a red precipitate will form
- if ketone present → solution remains blue
how can you test for aldehydes using tollens reagent
- if aldehyde present a layer of silver will form on the walls of the test tube
- if ketone present → remains colourless
how can reactions that produce carbonyl compounds from alcohols be reversed
oxidation reactions which produce carbonyl compounds from alcohols can be reversed via reduction reactions
what reducing agent can be used to reverse the oxidation of alcohols
lithium tetrahydridoaluminate (III) aka aluminium hydride (LiAlH4) is used as a reducing agent
- example of nucleophilic addition reaction
what solvent does the reduction reaction used to reverse the oxidation of alcohols require and why
- LiAlH4→ very reactive
- will react violently with water and alcohols
- so reaction is carried out in dry ether solvent
recall the steps of the reduction reaction used to reverse the oxidation of alcohols
- LiAlH4 provides the H:- nucleophile
- first the salt is formed
- then dilute acid is added to release the alcohol from the salt
- reducing agent represented by [H] in the equation
write the equation using displayed formula of the reduction of aldehydes to form an alcohol using the example of ethanol
show the mechanism for the production of a hydroxynitrile from an aldehyde- what kind of reaction is it
nucleophilic addition can also occur with the :CN- nucleophile
causes carbon chain to be extended by one carbon atom
what substance is usually used to provide the nucleophile instead of HCN in the mechanism to produce a hydroxynitrile and why
- KCN (potassium cyanide) is often used to provide the nucleophile instead of hydrogen cyanide (HCN)
- bc HCN is hard to store
- is a toxic gas
- reacts to produce dangerous by-products
also because theres a strong bond between H and CN whereas for KCN theres weak bonds so gives a greater supply of CN- ions
what do hydroxynitriles contain
hydroxynitriles contain a chiral carbon centre so optical isomers of the product are produced
why does the mechanism to form hydroxynitriles from aldehydes cause different enantiomers to be produced
the :CN- nucleophile can attack from either above or below the planar double bond
what is the iodoform test used to identify
- uses triiodomethane
- used to identify CH3CO- groups
- can be used to identify aldehydes and ketones
how is the iodoform test carried out
- iodine solution added to unknown sample
- then sodium hydroxide solution is added until the colour created by iodine is removed
what does a positive result for the iodoform test look like
positive result (ketone or aldehyde present) → pale yellow precipitate formed which is known as triiodomethane (CHI3)
what can the 2,4-dinitrophenylhydrazine test be used to test for
can be used as a test for the carbonyl functional group
what occurs when ketones or aldehydes are reacted with 2,4-DNPH
when aldehydes and ketones are reacted with 2,4-DNPH → yellow, orange or reddish-orange precipitate will form
What is the mtp of carbonyl compounds
carbonyl compounds have a sharp melting point → melt over narrow range of temps
how can mtp be used t identify carbonyl compounds
- 2,4-DNPH added to the compound so that a precipitate forms
- the solid is then purified by recrystallisation (crystalise again)
- mtp of the pure crystals can be formed and compared to the mtp of all 2,4-dinitrophenylhydrazones of all common aldehydes and ketones in a data booklet
what is the functional group for carboxylic group
carboxylic acids can be identified by the -COOH functional group
also contains a carbonyl group C=O and an -OH acid group
what is the suffix for carboxylic acids
suffix -anoic acid
draw an example of a carboxylic acid with 2 carbons
what are the two ways carboxylic acids can be produced
- carboxylic acids can be prepared by oxidising primary alcohols or aldehydes under reflux using potassium dichromate (VI)
- can also be produced by the hydrolysis of nitrile compounds
what allows carboxylic acids to form hydrogen bonds
-COOH functional group allows carboxylic acids to form H bonds between each other
why are carboxylic acids soluble in water
they can also form hydrogens bonds with water → meaning they’re soluble in water
what causes the trend in solubility of carboxylic acids
- as chain length inc, solubility dec as CH2 groups don’t form hydrogen bonds with water
- only form london forces with water
why do carboxylic acids have even higher mtp and mpts than alcohols, aldehydes and ketones
C=O and O-H parts of the functional group of carboxylic acids can form hydrogen bonds between molecules
draw an example of hydrogen bonding between 2 carboxylic acid molecules
recall the reactions that carboxylic acids can undergo
reduction
neutralisation with bases
acylation
esterification
what can carboxylic acids be reduced to and with what reducing agent
reducing agent LiAlH4 can be used to produce primary alcohols from carboxylic acids
LiAlH4 provides the H:- nucleophile
how are carboxylic acids reduced to primary alcohols
- carboxylic acids are first reduced to aldehydes
- however, aldehydes cant be isolated from this reaction as they’re more reactive than carboxylic acids
- so they’re immediately reduced further to primary alcohols
draw the displayed formula for the reduction of carboxylic acids to a primary alcohol using the example of ethanol
what is the reaction of carboxylic acids and a base
produced salt and water
carboxylate salt
R-COONa + H2O
R-carbon chain
what is a carboxylic acid derivative
carboxylic acids have derivative molecules where the -OH group is replaced by another group
what is acyl chloride an example of
Acyl chlorides are an example of a derivative of carboxylic acid
why do acyl chlorides react violently
that reacts violently due to the very polar -COCL group
draw an example of the acyl chloride functional group
how can acyl chlorides be produced from carboxylic acids
acyl chlorides can be produced by the reaction between carboxylic acids and phosphorus (V) chloride
write the equation for the reaction to produce an acyl chloride from ethanoic acid
how can esters be produced from a carboxylic acid + conditions
carboxylic acids can react with alcohols in the presence of a strong acid catalyst to form esters
- conc. sulfuric acid can be used as an acid catalyst
- this reaction is called esterification
- carried out under reflux
using the example of methanol and ethanoic acid, draw the reaction to produce an ester using displayed formula
what is the simplified steps of producing an ester from an alcohol and carboxylic acid
remove -OH group from the acid and the hydrogen from alcohol to make water
then join the acid and alcohol together
what are the properties of esters
- esters are sweet smelling compounds
- used in food flavourings and perfumes
- they have low bpts and are good solvents for polar molecules
how are esters named
beginning of alcohol name
beginning of carboxylic acid name with -anoate ending
e.g an ester made from ethanol and propanoic acid would be called ethyl propanoate
how are acyl chlorides named
have the suffix -oyl chloride, stem of their name representing the longest chain of carbon atoms
e.g butanoyl chloride
what is the functional group of esters
-COO
what is the functional group of esters
-COO
acyl chloride + water
carboxylic acid
acyl chloride + alcohol
ester
acyl chloride + ammonia
amide
draw the functional group for an amide
acyl chloride + amine
N-substituted amide
what is an N-substituted amide
N-substituted amide has another carbon group attached to the nitrogen (instead of hydrogen)
Through what mechanism do acyl chlorides react
- acyl chlorides react via nucleophilic addition- elimination reactions
- the addition of a nucleophile in these reactions leads to the elimination of a product under aqueous conditions
how can you convert esters back into alcohols
ester hydrolysis
convert esters back into alcohols and carboxylic acids which is done by adding water
what is the reaction of esters and water under acidic conditions- draw the equation
reverse reaction → turns ester back into alcohol and carboxylic acids by adding water
what is the reaction of esters and a base (alkaline conditions)- draw the equation
- carboxylic acid produced can react further with a base to form a salt
- this process is called saponification
- these salts have hydrophobic and hydrophilic properties
how are condensation polymers formed
formed when a water molecule is removed from the species of a reaction
how are polyesters formed
from a reaction between a dicarboxylic acid and a diol which produces an ester linkage, -C(double bond)OO-
why are polyesters biodegradeable
- polyesters can be broken down through hydrolysis due to the polarity
- therefore they’re biodegradable
- can be broken down easily by naturally occurring water or moisture
- so they can be put into landfill where they will break down gradually
what is solid sodium carbonate used to test for
acids
what is PCL5 used to test for
OH group
why may the product for the reaction of a molecule with a chiral centre lack optical activity
the intermediate is planar so the attacks can happen from above or below
leading to the formation of a racemic mixture
which compound is oxidised by ammoniacal silver nitrate in the test using tollens reagent
aldehydes
how do you test for a carbonyl compound using tollens reagent
is an oxidation reaction
add silver nitrate to a test tube
add 2M dilute ammonia solution drop wise until the precipitate dissolves
then add 10 drops of the carbonyl compound and shake
warm the contents in a water bath
silver mirror forms in the presence of an aldehyde
how do you carry out the test for aldehydes using benedicts solution
is an oxidation reaction
add a few drops of the aldehyde to water in a boiling tube
add the same volume of benedicts solution as the volume water
colour change blue to brick red
what are the conditions and reagents for oxidising an alcohol
reflux
add dilute sulfuric acid to a warm solution of potassium dichromate
(acidic conditions)
How can a polyester be produced
molecule with OHand COOH group
Or dicarboxylic and diol
What additional force can carboxylic acids form
can form dimers that are even stronger than all of the intermolecular forces because all of the Os and Hs in the COOH functional group can have a slight charge
Carboxylic acids can form London forces, hydrogen bonds and dimers
