3.5 isomerism/carbonyl compounds

Question 1

what is optical isomerism?

Answer 1

a type of stereoisomerism
optical isomers have a chiral (asymmetric) carbon which means it has 4 different groups attached to it.

Question 2

what are enantiomers/ optical isomers?

Answer 2

**a pair of non-superimposable mirror images **
different molecules formed as you can arrange the groups around the chiral carbon to form 2 different molecules.
if molecules can be superimposed, they are achiral so there’s no optical isomerism.

Question 3

how do you draw optical isomers?

Answer 3

locate the chiral carbon
draw 1 enantiomer in a tetrahedral shape with the chiral carbon in the middle
put a mirror line next to it and draw the mirror image

Question 4

are optical isomers optically active? what does this men?

Answer 4

they rotate plane polarised light

Question 5

what is plane polarised light?

Answer 5

normal light vibrates in all directions . if normal light is passed through a polarising filter, it becomes plane polarised light which means all the light is vibrating in the same plane

Question 6

how can you distinguish between enantiomers?

Answer 6

use plane polarised light
the different enantiomers rotate the plane of plane polarised light in different directions
+ enantiomer rotates it right
- enantiomer rotates it left
can use a polarimeter to analyse effect of optical isomers on plane polarised light

Question 7

what is a racemate/racemic mixture?

Answer 7

contains equal quantities of each enantiomer of an optically active compound. enantiomers cancel each others light-rotating effect so racemates dont show any optical activity

Question 8

question for this

Answer 8

chemists often react 2 achiral things together and get a racemic mixture of a chiral product because when 2 molecules react, theres normally an equal chance of forming each of the enantiomers. examples on page 172-173

Question 9

what are aldehydes and ketones? how are they different?

Answer 9

both carbonyl compounds as they both contain C=O (carbonyl functional group). but, this is in a different position in both

Question 10

why arent ketones easy to oxidise?

Answer 10

only way to oxidise would be to break a carbon carbon bond

Question 11

how can we reduce ketones and aldehydes into alcohols?

Answer 11

NaBrH4 (sodium tetrahydridoborate (III))
in equations, [H] is used to indicate a hydrogen forming a reducing agent

Question 12

what is the mechanism for the reduction of aldehydes and ketones into alcohols?

Answer 12

nucleophilic addition
H- ion from reducing agent acts as a nucleophile and adds onto d+ of carbon atom from carbonyl group

Question 13

what happens in nucleophillic addition?

Answer 13

C=O bond is polar meaning C in carbonyl group is d+ so attracts - charged lp on the H-
H- attacks Cd+ and donates its lp forming a bond w carbon
carbon can only have 4 bonds so one C=O breaks forcing a lp from this back onto O
O donates lp to H+ (from H2O/ weak acid)
alcohol produced

Question 14

what are aldehydes reduced to? ketones?

Answer 14

primary alcohols
secondary alcohols

Question 15

what are hydroxynitriles?
how are they named?

Answer 15

molecules containing a hydroxyl (OH) group and a nitrile (CN) group
nitrile group is most important so suffix is nitrile and carbon it is attached to is always 1. hydroxy prefix
eq 2-hydroxybutanenitrile (put the e back in after the butan)

Question 16

how are hydroxynitriles produced?

Answer 16

react aldehyde/ ketone with potassium cyanide followed by dilute acid

Question 17

what is the mechanism for producing hydroxynitriles?

Answer 17

nucleophilic addition
KCN is an ionic compound which dissociates into K+ and CN- in water
CN- attacks C^d+ and donates pair of electrons to form bond with C
pair of electrons from C=O is pushed onto O
O bonds to H+ (from dilute acid) to form hydroxyl group and hydroxynitrile is produced

Question 18

how can a nucleophillic attack of cyanide ions on aldehydes/ unsymmetric ketones form a racemic mixture are hydroxynitriles?

Answer 18

double bonds such as C=O are planar so nucleophiles can attack from above or below the plane resulting in diff enantiomers. a racemic mixture is formed as attack from each side is equally likely

Question 19

dissociation of carboxylic acids

Answer 19

weak acids so partially dissociate into a carboxylate ion and a H+ ion in water
reversible reaction but eqm lies to left as most molecules dont dissociate

Question 20

reaction of carboxylic acids with carbonates

Answer 20

react with carbonates (CO3^2-) or hydrogen carbonates (HCO3^-) to form a salt, CO2 and H2O
in these reactions, CO2 fizzes out of the solution (there is effervescence)

Question 21

what is an esterification reaction?

Answer 21

heat a carboxylic acid with an alcohol in the presence of a strong acid (eg H2SO4, HCL or H3PO4) catalyst and you get an ester

Question 22

how do you name esters?

Answer 22

first bit is from the alcohol and the second bit is from the carboxylic acid
eg ethanoic acid and methanol make methyl ethanoate

Question 23

useful properties of esters

Answer 23

sweet smell- gluey sweet for smaller ones and pear drops for
larger ones- useful in perfumes and flavourings

polar liquids so lots of organic compounds dissolve in them

low bp so evaporate easily from mixtures- good solvents in glues and printing inks

plasticisers

Question 24

what is hydrolysis? how many types are there?

Answer 24

when a substance is split up by water
2- acid catalysed and base catalysed

Question 25

what is acid catalysed hydrolysis?

Answer 25

splitting ester into acid and alcohol reflux ester with a dilute acid eg hcl of h2so4 ester splits back into the carboxylic acid and alcohol it was made from see page 184 for example

Question 26

what is base catalysed hydrolysis?

Answer 26

reflux ester with a dilute alkali eg naoh oh- ions from base react w ester and you get a carboxylate ion and an alcohol see page 185 for example the - carboxylate ions bond w the +ion from the base eg Na+ to form salts

Question 27

what are fatty acids?

Answer 27

long chain carboxylic acids fatty acids combine w glycerol to make fats and oils (see page 185) fatty acids can be saturated or unsaturated most of a fat/ oil is made from fatty acid chains so it is these that give them many of their properties

Question 28

what is glycerol?

Answer 28

propane-1,2,3-triol

Question 29

why are fats solid at room temp? why are oils liquids at room temp?

Answer 29

fats have mainly **saturated** hydrocarbon chains so fit neatly together increasing vdw forces between them so higher temp needed to melt them oils have **unsaturated** hydrocarbon chains and double bonds mean chains are bent and dont pack together well which decreases the effect of vdw forces so easier to melt

Question 30

how can you hydrolyse fats and oils? what is produced?

Answer 30

heat them with NaOH- type of base hydrolysis OH- ions react with the fat/ oil to form a carboxylate ion and alcohol the alcohol formed is glycerol and the carboxylate ions combine with Na+ ions to form a sodium salt the sodium salt produced is soap

Question 31

how can you convert soap back into a long chain carboxylic acid (fatty acid)?

Answer 31

add an acid eg HCL H+ ions displace Na+ ions in the salt to form a carboxylic acid which releases a free Na+ ion CH3(CH2)16COO-Na+ + H+ -> CH3(CH2)16COOH + Na+

Question 32

what is biodiesel? how is it made?

Answer 32

react oils with methanol using a strong alkali eg KOH or NaOH catalyst you get a mixture of methyl esters of long chain fatty acids (biodiesel)

Question 33

is biodiesel carbon neutral?

Answer 33

can be thought of as carbon neutral as crops absorb same amount of CO2 when they grow as they produce when theyre burned but, energy is used to make fertiliser to grow the crops, plant and harvest crops and convert the oil this energy is from fossil fuels so process isnt C neutral overall

Question 34

what are acyl chlorides? functional group? general formula? how do you name them?

Answer 34

type of carbonyl compound COCl functional group CnH2n-1OCl general formula all end in oyl chloride and carbon atoms are numbered from end w acyl functional group eg pentanoyl chloride

Question 35

reaction of acyl chlorides with water

Answer 35

react vigorously w cold water to produce a carboxylic acid eg ethanoyl chloride + H2O -> ethanoic acid + HCl

Question 36

reaction of acyl chlorides with alcohols

Answer 36

react vigorously with alcohols at room temperature to produce an ester it is an irreversible reaction that is an easier and faster way to produce an ester than esterification eg ethanoyl chloride + methanol -> methyl ethanoate + HCl

Question 37

reaction of acyl chlorides with ammonia

Answer 37

react vigorously with ammonia at room temp to produce an amide eg ethanoyl chloride + NH3 -> ethanamide + HCl

Question 38

reaction of acyl chlorides with primary amines

Answer 38

react vigorously with primary amides at room temp to produce an N-substituted amide eg ethanoyl chloride + methylamine -> N-methylethanamide + HCl

Question 39

what is the nucleophilic addition- elimination mechanism?

Answer 39

the reactions involving acyl chlorides step 1: nucleophile adds to acyl chloride and displaces cl- step 2: hydrogen leaves to create an acyl chloride derivative see page 189 for steps **(make sure you learn the steps)**

Question 40

what are acid anhydrides?

Answer 40

compounds that react in a similar way to acyl chlorides made of 2 identical carboxylic acid molecules which are joined together by an oxygen with the carbonyl groups on either side the O comes from OH group of 1 of the COOH and the other OH group and the spare H are released as water eg ethanoic acid + ethanoic acid -> ethanoic anhydride + water

Question 41

reaction of acid anhydrides with water

Answer 41

eg ethanoic anhydride + water -> ethanoic acid x2 almost same as reactions for acyl chloride but less vigorous, carboxylic acid not HCl mechanism is nucleophilic addition-elimination

Question 42

reaction of acid anhydrides with alcohols

Answer 42

eg ethanoic anhydride + methanol -> methyl ethanoate + ethanoic acid almost same as reactions for acyl chloride but less vigorous mechanism is nucleophilic addition-elimination

Question 43

reaction of acid anhydrides with ammonia

Answer 43

eg ethanoic anhydride + NH3 -> ethanamide + ethanoic acid almost same as reactions for acyl chloride but less vigorous mechanism is nucleophilic addition-elimination

Question 44

reaction of acid anhydrides with amines

Answer 44

eg ethanoic anhydride + methyl amine -> N-methylethanamide + ethanoic acid almost same as reactions for acyl chloride but less vigorous mechanism is nucleophilic addition-elimination

Question 45

what is aspirin? how do you make it?

Answer 45

an ester react salicylic acid (which has an alcohol group) with either ethanoic anhydride or ethanoyl chloride eg salicylic acid + ethanoic anhydride -> aspirin + ethanoic acid

Question 46

why is ethanoic anhydride used more in industry than ethanoyl chloride?

Answer 46

produces a carboxylic acid or carboxylate salt as a byproduct but ethanoyl chloride produces dangerous HCl fumes or chloride salts cheaper less corrosive as a reagent reactions are more controllable but reacts more slowly

Question 47

what are the steps of purifying organic compounds?

Answer 47

separation solvent extraction drying agent washing distillation redistillation recrystallisation testing purity

Question 48

practicals in this topic

Answer 48

making aspirin (purifying organic compounds)

Question 49

describe separation when purifying organic products

Answer 49

use to remove any soluble impurities from an insoluble product organic layer and aqueous layer (contains water soluble impurities) are immiscible so separate into 2 distinct layers. open the tap to run each layer off into a separate container organic layer is less dense than aqueous layer so floats on top and water insoluble impurities dissolve in lower aqueous layer

Question 50

describe the solvent extraction step of purifying an organic product

Answer 50

if product and impurities are all dissolved in solution together, you can use solvent extraction which is another form of separation. vigorously shake impure product with immiscible solvent so they temporarily mix product needs to be more soluble in the added immiscible solvent that the one it was initially dissolved in if this is the case, product will dissolve in added solvent and separate from the solution containing impurities solvent containing product can then be run off using separating funnel

Question 51

describe the step of purifying an organic product that involves using a drying agent

Answer 51

if you use separation, organic layer will end up w trace amounts of water so it has to be dried add an anhydrous salt eg MgSO4 or CaCl2 which act as drying agents to bind to water present and become hydrated when you first add it, clumps will form meaning you need to add more. enough is added when it looks like a snow globe filter to remove solid drying agent

Question 52

what is the washing step of purifying an organic product

Answer 52

product of a reaction can be contaminated w unreacted agents/ unwanted side products which can be removed by washing product eg (page 194 and tip)

Question 53

how do you carry out the distillation step of purifying an organic product?

Answer 53

heat mixture in distillation apparatus and diff substances evaporate out in order of bp thermometer placed in neck of condenser and shows bp of substance evaporating if product has lower bp than starting material, reaction can be heated in distillation apparatus so product evaporates as it forms. if starting materials have a higher bp than product, so long as temp is controlled, they wont evaporate out

Question 54

how do you carry out the redistillation step of purifying an organic product?

Answer 54

mixture containing volatile liquids can be purified by redistillation if product and impurities have diff bp, this can be used to separate them. use same method but this time youre heating impure product not the reaction when liquid boils, place flask at open end to collect product when thermometer shows temp is changing, put another flask there as diff liquid is about to be delivered

Question 55

how do you carry out the recrystallisation step of purifying an organic product?

Answer 55

- Add minimal amount of hot solvent to make saturated solution - filter warm - place in ice bath- solubility of product will fall as it cools (impurities stay in solution as theyre in smaller quantities than product so take longer to crystallise out) - filter under pressure - wash and dry

Question 56

how do you test the purity of an organic product?

Answer 56

use melting point apparatus to accurately determine the melting point of an organic solid pack small amount into capillary tube and place in heating element then heat slowly near to the melting point. increase until it melts can look up melting point of a substance in data books and compare to your measurements (impure- lower mp and over a range)

Question 57

what is stereospecificity?

Answer 57

the active site of an enzyme is stereospecific which means only 1 enantiomer will fit into the active site

Question 58

+ and - of selling racemate drugs

Answer 58

+ racemate doesnt have to be separated so lower cost and difficulty of production - only 50% of drug administered has the desired effect so higher quantity needs to be administered the other enantiomer could cause unwanted side effects- have to be rigorously tested. eg thalidomide