module 6: carboxylic acids and nitrogen compounds

Question 1

are carboxylic acids weak/strong

Answer 1

weak because they’re only partially ionised in solution

Question 2

are carboxylic acids water soluble

Answer 2

less than 6C per molecule = water soluble

Question 3

how can you test for a carboxylic acid

Answer 3

react with a carbonate(eg. Na2Co3 or NaHCO3), a carboxylic acid would produce CO2

Question 4

how can an acyl chloride be formed from a carboxylic acid

Answer 4

carboxylic acid + SOCl2 –> acyl chloride + SO2 + HCl

Question 5

outline recrystallisation

Answer 5

1. Add minimal hot solvent to produce a saturated solution of the impure compound.
2. Filter the hot solution. Remove insoluble impurities by using gravity filtration.
3. Let the solution cool. As the solution cools down, the solubility of the compound decreases.
4. Crystals form as the solubility of the compound decreases. As the concentration of impurities is much lower, they take longer to from crystals and hence will remain in solution.
5. crystals are removed by using filtration under reduced pressure and then rinsed with cold solvent. Finally they are let dry.

Question 6

how are amines classified

Answer 6

based on the number of R groups on the nitrogen atom

Question 7

what are the properties of amines

Answer 7

the nitrogen atom in ammonia and amine molecules has a lone pair, allowing it to accept a proton

so they can act as bases in aqueous solutions by donating the lone electron pair to a proton and forming a dative bond

Question 8

are amines or ammonia more basic

Answer 8

the more alkyl groups substituted onto the N atom in place of H atoms, the more electron density that is pushed on to the N atom
so more alkyl groups increases the electron density of N’s lone pair, forming a stronger base

amines are more basic

Question 9

are aromatic amines or ammonia more basic

Answer 9

on aromatic amines the lone pair on N is partially delocalised into the benzene ring, reducing the electron density on the N atom and making it a weaker base

ammonia is more basic

Question 10

how does NH3 react with haloalkanes

Answer 10

nucleophilic substitution

if excess haloalkane is present, then the H’s on the N can be successively replaced by the haloalkanes R group until a quaternary ammonium salt is formed

if excess amine is present, there is only one substitution

Question 11

what is the mechanism of reaction for NH3 and haloalkanes

Answer 11

nucleophilic substitution

Question 12

how are primary aromatic amines formed

Answer 12

1. reduce nitrobenzene by reacting with tin and conc HCl
   (ignore 2nd bit)

Question 13

how are primary aliphatic amines formed

Answer 13

NH3 + haloalkanes
haloalkanes + excess, hot ethanolic ammonia
- forms mixtures and not usually primary amines

OR

reducing nitrile compounds
a two step process
1. form a nitrile from a haloalkane
2. react nitrile with a reducing agent; LiAlH4/Ni+H2

Question 14

how do amino acids react

Answer 14

the basic amine group reacts with acids to make salts; to is protonated in neutral solutions and acids

the COOH group is deprotonated in neutral solutions and bases

Question 15

what are the types of stereoisomers

Answer 15

geometrical (E/Z)
optical

Question 16

how do chiral molecules exist

Answer 16

as two enantiomers (optical isomers), they are non-superimposable images of each other (like hands!!!)

Question 17

how is a chiral centre marked in a drawing

Answer 17

with an asterisk

Question 18

how do you do a 3D drawing of a tetrahedral molecule

Answer 18

2 solid lines - same plane as paper
1 dashed wedge - bond receding behind paper
1 solid wedge - bond coming out of paper

Question 19

what are the properties of optical isomers

Answer 19

they are optically active and rotate plane-polarised light
one enantiomer will rotate in a clockwise direction, whilst the other routes it the same amount anticlockwise

enantiomers can sometimes be referred to as the D/L isomer

Question 20

outline filtration under reduced pressure

Answer 20

a solid product can be separated from liquid impurities via reduced pressure filtration
- reaction mixture is poured into a Büchner funnel with a piece of filter paper in it
- the funnel is on top of a sealed sidearm flask which is connected to a vacuum line, causing it to be under reduced pressure
- reduced pressure causes suction through the funnel, making the liquid pass into the flask quickly and leaving behind dry crystals of the product

Question 21

what is the general structure of an amide

Answer 21

Question 22

what is phenol reacted with to form 2-nitrophenol/4-nitrophenol vs 2,4,6-nitrophenol

Answer 22

2-nitrophenol/4-nitrophenol: dilute HNO3
2,4,6-nitrophenol: conc. HNO3

Question 23

what is a racemic mixture

Answer 23

a mixture with equal quantities of two enantiomers

Question 24

how can a polyamide be formed

Answer 24

diamine + dicarboxylic acid
diamine + dioyl chloride
amino acids

Question 25

how can a polyester be formed

Answer 25

diol + dicarboxylic acid
diol + dioyl chloride
hydroxycarboxylic acids

Question 26

how do you name an addition polymer

Answer 26

poly(alkEne)

Question 27

what are the features of C13 NMR spectra

Answer 27

• sharp signals
• signal heights aren’t proportional to the number of carbon atoms in a single molecular environment
• C atoms in different environments will give resonances with diff chemical shifts in a spectrum
• the standard reference point used is tetramethylsilane (TMS)

Question 28

how is a carbon’s environment determined in C13 NMR

Answer 28

• the groups surrounding the carbon are looked at
• the signals will depend on what groups are attached
• the signal shows will be the higher one if there are two fun groups attached

Question 29

outline how thin-layer chromatography works

Answer 29

separates components by aDsorbtion
1. draw pencil line near the bottom of the plate and put a spot of the mixture to be separated on the line
2. dip the bottom of the plate into a solvent
3. as the solvent spreads up the plate the different substances move with it, but different distances, so they seperate out
4. when the solvents nearly reached the top of the plate, the plate is removed and the distance that the solvent moved is marked in pencil

Question 30

how can retention time be found from a gas chromatogram

Answer 30

the time where it peaks, these can then be looked up in a reference table to identify the substances present

Question 31

how can a gas chromatogram show proportions of components in a mixture

Answer 31

the area under the peak will be proportional to the relative amount of substance in the original mixture

Question 32

how can concentrations of substances be found from a gas chromatogram

Answer 32

1. create a series of different concentrations of your analyte (the substance you’ve chosen to detect for your calibration)
2. one at a time, inject your standard solution into a gas chromotography instrument and detect the result
3. calculate the area under the graph for each standard solution
4. plot this on a graph of area against concentration
5. join the points to create an external calibration curve

Question 33

what factors can affect retention times

Answer 33

1. solubility: a more soluble substance will spend more time dissolved and will take longer to travel through the tube to the detector
2. boiling point: high boiling point means that the substance will spend longer condensed as a lid on the tube than a gas and so will take longer to travel through the tube than if it had a lower bp
3. temp of gas chromatography instrument: high temp means the substance will be evaporated for longer and move along the tube quickly; this hornets retention time for ALL substances in the tube

Question 34

how do you interpret proton NMR

Answer 34

TMS is used for the reference peak
H nuclei with different neighbouring atoms (chemical environments) absorb at slightly different field strengths
- spin-spin coupling will split peaks into n+1 where n is the number of H on the neighbouring C atom

Question 35

what solvent is used for proton NMR spectroscopy

Answer 35

NMR spectra are always used with molecules in solution but things like water and ethanol would form more peaks

so the H in the solvent are replaced with deuterium nuclei which don’t absorb the radio weave energy and so don’t form a peak

Question 36

how can OH and NH protons be identified with proton NMR

Answer 36

these have v variable NMR shifts
1. run two spectra of the molecule, one of which has an some D2O added
2. if any OH or NH protons are present, they’ll swap with the deuterium and the peak will disappear

Question 37

how do you calculate Rf

Answer 37

distance component travelled / distance solvent travelled

Question 38

outline the use of two deuterated compounds in HNMR spectroscopy

Answer 38

CDCl3 is used as a solvent
D2O is used to identify OH or NH protons

Question 39

what happens when an amino acid reacts with acid

Answer 39

NH3 and + ON NITROGEN

Question 40

how do you find the Mr of a polyester

Answer 40

Mr of whole monomer x no. molecules
then subtract H2O (or whatever) multiplied by no. molecules - 1