ocr chemistry a-level unit 3 (ring,polymer & analysis) revision notes

8/12/2019 ocr chemistry a-level unit 3 (ring,polymer & analysis) revision notes

1/26

Module 3 - Rings, Polymers and Analysis

1.1 Rings, Acids and Amines

Structure of Benzene

Benzene C6H6; colourless liquid, sweet odour, highly flammable, carcinogenic.

Formed by natural; volcanoes and forest fires. Human; oil, petrol or cigarette smoke.

ses; starting material for synthesis of aromatic compounds, e!plosive, detergents.

a) Keul! "tructure proposed by #ekul$ had ring of carbons with alternating single and double

bonds %double bonds are shorter than single bonds&

1. Pro"lems

'here are problems with this structure. Firstly, all of the C(C bond lengt#sin ben)eneare the same and are in between the length of a C(C and a C*C. "econdly, ifben)ene contained double bonds it would undergo addition reactions%likealkenes&. However, ben)ene actually undergoes substitution rather than addition

$. %nt#al&ies of #ydrogenation

'he enthalpy of hydrogenation of cyclohe!ene is (+- k mol(+

/f ben)ene had alternating double and single bonds, we would e!pect its enthalpy ofhydrogenation to be 0 ! (+- * (06- k mol(+

However, its actual enthalpy of hydrogenation is only (-1 k mol(+

2en)ene is +3 k mol(+lower in energy %more stable& than the hypothetical structurecontaining alternating double and single bonds

")'elocalisation

of electrons 'he accepted structure

for ben)ene is a planar%flat& ring of 6 carbon atoms,each of which is also

bonded to an H and othercarbon atoms, trigonalbond angle +-o.

1

4nergy


2/26

5 electrons and 0 bonded, sigma bond. 4lectron left in p orbital above and below theplane of the carbon atoms.

'he p orbitals overlap producing a system of bonds spread over all 6 carbonsresulting in a ring of electron density above and below with delocalised electrons.

c) Reacti(ity "tability makes it harder to take part in addition as the electrons will need to bond

with the atom being added resulting in product with less stability %not energeticallyfavourable&. on(reactive; strong acids%HCl&, halogens or bromine water%decolourise&

2en)ene undergoes substitution reactions rather than addition to maintaindelocalisation of electrons and stability.

d) Reactions 'he high electron density in the (bonds make ben)ene attractive to electrophiles

%electron pair acceptors&.

'he mechanism for the following reactions of ben)ene is electroilic su"stitution

1. itration

4quation *++ 3 *+/$ $

7eagents conc H80and conc H"85

Conditions 93-oC

:eneration of electrophile H80 H"858 H"85( H8

$. alogenation

4quation *++ *l$ *+/*l *l

7eagents Cland halogen carrier; Fe, FeCl0or ?lCl0%catalyst&

:eneration of electrophile Cl ?lCl0Cl ?lCl5(

2


3/26

") *yclo#e0ene

c) *om&arison it# alenes

?n alkene has a double bond, localised electrons.

2en)ene has lower electron density than alkene so not able to polarise bromine sothe electrophile needs to be generated.

'he delocalised electrons are not easily disrupted so the activation energy forben)ene is higher than for an alkene.

P#enola) Pro&erties


4/26

*ar"onyl com&oundsa) 6ntroduction

?ldehydes %al& and ketones %one, position number like double bond& are carbonylcompounds

'hey contain the carbonyl group C*8

Bouble bond different to alkene as o!ygen is more electronegative than carboncreating dipole.

") 0idation


5/26

'he recrystalli)ed compound has to be left to dry as water can have effect on themelting point.

$. ;ollen:s Reagent /t is a weak o!idising agent that is used to distinguish between aldehydes and

ketones. 'he compound to be tested is warmed with 'ollenDs reagent. =ade by>

+. aq sodium hydro!ide%a8H& added to aq silver nitrate %?g80& until brownprecipitate of silver o!ide %?g8& formed.

. Bilute aq ammonia is added to precipitate until it Eust dissolves ?mmoniumsilver nitrate %'ollens reagent&.

?ldehydes are o!idised by 'ollenDs reagent which is reduced to silver metal

*3* 78 *3* 0idation

Ag4a) e- Ag4s) Reduction

/f aldehyde present then a silver mirror or silver-grey solid forms.

#etones do not react with 'ollenDs reagent because they are not easily o!idised.

*ar"o0ylic acids

Carbo!ylic acids contain the functional group C88H on the end of a chain.

a) Solu"ility

'hey are soluble in water because the C*8@C(8H can form hydrogen bond to watermolecules 8(H.

?s the chain length increases the solubility decreases as chain non(polar.

") Reactions of car"o0ylic acids 'hey are weak acids %Hdonors&. 'he acidic H is in the C88H group e.g.

'hey take part in typical acid reactions forming salts known as carbo!ylate %ate&

*3**3*- %note reversible reaction so not &

Metals

*3* a *3*a 2$ Fi))ing seen

4thanoic acid sodium ethanoate "odium dissolves

*ar"onates

$*3* a$*3 $*3*a $ *$ Fi))ing seen

Carbonate dissolves

Bases

*3* a *3*a $

%sters 4sters contain the functional group C887 on the end of a chain

4sters can be made in two ways> carbo!ylic acid alcohol or acid anhydride alcohol

4sters are sweet smelling and are used as flavourings and perfumes in food.

a) %sterification of car"o0ylic acid it# alco#ol

5


6/26

Conditions> 7eflu! with concentrated H"85 %acts as a catalyst&

4ster and water.

*3* *$/*3**$/ $4thanoic acid ethyl ethanoate

") %sterification of acid an#ydride it# alco#ol :ently heating acid anhydride with alcohol.

=uch better yield.

?cid anhydrides can be thought of as molecules of acid that have lost a molecule ofwater

4*3*$*)$ *3 *3*$**3 *3*$*


7/26

CH,CH,CH,CH,CH,CH,CH0 CH, CH, CH, CH, CH, CH, CH, CH, CH, CH8

8

CCH,

CH, C

8

8 CH,CH,CH,CH,CH,CH,CH,CH,CH,CH,CH0 CH, CH, CH, CH, CH, CH,

CH, C

8

8 CH,CH,CH,CH,CH,CH,CH,CH,CH,CH,CH0 CH, CH, CH, CH, CH, CH,

? fatty acid long chain carbo!ylic acid, saturated or unsaturated.

'he shorthand formula for a fatty acid>+. First number indicates number of carbon atoms. "econd number indicates number of double bonds0. umber in brackets indicates the position of double bonds. 4.g +1> %G,+&

CH, C

8

8 HCH,CH,CH,CH,CH,CH,CH,CH,CH,CH0 CH, CH, CH, CH, CH, CH,

:lycerol is propane(+,,0(triol

/n a triglyceride the 0 fatty acids do not have to be the same

c) Saturated and unsaturated fats

'riglycerides can be saturated or unsaturated depending whether they are derivedfrom fatty acids that are saturated or unsaturated.

'he presence of double bonds means thatunsaturated fatty acids can have cis and transisomers %stereoisomer& due to restricted rotation.

Food containing saturated fatty acids increasesthe risk of heart disease. nsaturated fatty acidsare thought to be better for health.

Cis cannot pack closely together and e!sist asliquid.

'rans linear and can pack closely together andhave higher melting point.

'he presence of trans fatty acids in the diet raises the level of ABA %badD& cholesterol

and reduces the level of HBA %goodD& cholesterol. 'his increases the risk of coronaryheart disease and strokes

d) Biodiesel 2iodiesel consists of the esters of fatty acids %methyl, ethyl&

2iodiesel can be made from cooking oil or crops such as palm, rapeseed or soyabean.

=ade by process known as transestrification.

'riglyceride in fats@oil reacted with methanolin presence of sodium%potassium&hydro!ide acting as a catalyst

'he by(product glycerol produced sold to pharmaceuticals or cosmetics. /mprovingatom economy.

7
http://dostructure%28%27methanol.htm%27%29/http://dostructure%28%27methanol.htm%27%29/http://dostructure%28%27methanol.htm%27%29/


8/26

; 0 CH0 8H

CH,

CH

CH,

8H

8H

8H

8C

8

7

8C

8

7

CH,

CH

CH,

7

8

C 8 8C

8

7 CH0; 0

6a8H

Aminesa) introduction

Berived from ammonia. 'he hydrogen atoms from ammonia are replaced by alkylgroups%hydrocarbon chains&.

#nown for their unpleasant fishy odour and is a weak base.

/n amines, the has 0 bonding pairs and + lone pair so shape is pyramidal, bond

angle * +-

aming; butylamine, ( methlprophylamine, , diethylpropylamine.

") Amines as "ases

2ases are proton %H

& acceptors. ?mines can accept protons because of the lone pair on the

'his lone pair forms a co(ordinate bond%dative covalent bond& with an He.g.

1. Acid *3$ *l *33 *l-

=ethylamine methylammonium chloride

2. =ater *3*$$ $ *3*$3 -4thylamine ethylammonium hydro!ide

now has 5 bonding pairs so its shape is tetrahedral, bond angle +-G.3

c) Pre&aration of amines Iarming halogenalkanes gently with an e!cess of ammonia, using ethanol as

solvent.

7eagents e!cess ammoniaConditions dissolved in ethanol

*3*$*l 3 *3*$$ *l

Chloroethane ethylamine

'he HCl formed reacts with ammonia.

3 *l 9*l-

8


9/26


10/26

o 'hese reactions are used in formation of dyestuffs %soluble substances used for

staining or colouring fabrics&.

$ > Polymers and Synt#esis

Amino acids

a) introduction ?mino acids found in living things, protein as made from peptide, have the general

formula 7CH%H&C88H where 7 is an alkyl group

?mino acids contain an acidic functional group %(C88H& and a basic functional group

%(H&. 'his means they can act as both acids and bases. 'hey are soluble in both acid and base.

10


11/26

:lycine simplest amino acid as 7 group is H.

?ll amino acids, apart from glycine, show optical isomerism as the central C has fourdifferent groups attached to it

") %ffect of & ?t a pH value called the isoelectric point, amino acids e!ist as )witter ions. 'hese ions contain a full positive charge and a full negative charge, therefore the

charge in neutral. 'he C88H has lost Hand the Hhas gained H

?mino acids e!ist in this form in the solid state and have high melting points becausethere are strong ion(ion forces between the )witter ions

Bifferent 7 groups in (amino acids result in different isoelectric points

c) *ondensation ?mino acids can Eoin together to form dipeptides by elimination of water molecule

'his is known as condensation reaction.

Can for di, tri or polypeptide.


12/26

Heat under reflu! for 5 hours.

C88H groups and H0 due to all the H ions present.

Alaline #ydrolysis

7eagents> ?q a8H

Conditions> 9+--o

carbo!ylates %C88(groups&

e) Stereoisomerism

/somers

KKKKKKKKKKKKKKKKKKK

"tructural "tereoisomers/somers KKKKKK KKKKKKK

4@L isomers 8ptical isomers

"tereoisomers have the same structural formulae but different arrangement of atomsin space.

'here are two types of stereoisomerism> 4@L isomerism, which occurs in alkenes%double bonds&, and optical isomerism, which occurs when a carbon atom is attachedto four different groups %chiral carbon&.

1. %?@ isomers 4@L isomers occur in alkenes because of restricted rotation about a double bond.

12


13/26

For 4@L isomers to occur, the two groups at each of the double bond must be different

L(but((ene %cis& 4(but((ene %trans&

$. &tical isomers 'his occurs in chiral carbons %carbon attached to 5 different carbon compounds&.

'hey are non(superimposable mirror images of each other.

Chemically identical but rotate plane polarised light in different directions; one rotateslight clockwise and the other rotates light anti clockwise.

=i!ture containing equal amount of each isomer is known as racemic. o effect onplane(polarised light as they cancel each other out.

8nly optical isomers formed. aturally occurring amino acid are optically active, only one of the optical isomer is

biologically active %en)yme&.

3. Synt#esis of armaceuticals Compound with the single correct isomer>

+. 7isks for undesirable side effects are reduced.. Brug doses reduced

"eparation by; en)yme, chromatography or melting and boiling point.

"ynthesis of a single optical isomer of a pharmaceutical increases costs due to thedifficulty in separating the optical isomers.

=odern synthesis of a single optical isomer of a pharmaceutical is often carried out>o sing en)ymes as biological catalysts

o Chiral pool synthesis.

o se of transition element comple!es

Polymerisation

a) *ondensation &olymers For condensation polymerisation to occur>

+. ? monomer with different functional group. monomers with di different functional groups %diol, diamine, dicarbo!ylic&

13


14/26

'he functional groups react to form a link; the reaction also produces a smallmolecule, such as water

'he links in condensation polymers %ester, amide and peptide& can be hydrolysed byheating with dilute acid or alkali

Polyesters

'he monomers are a carbo!ylic acid and a alcohol

?n ester link is formed between an acid group and an alcohol group with a watermolecule eliminated

Terylene

=onomers > ben)ene(+,5(dicarbo!ylic acid and ethane(+,(diol

ses > lactic acid %(hydro!ypropanoic acid&

ses > biodegradable so used in food and drink packaging

14


15/26

Polyamides

'he monomers are a carbo!ylic acid and a amine

?n amide link is formed between an acid group and an amine group

Nylon-6,6

=onomers> he!ane(+,6(dicarbo!ylic acid and +,6(diaminohe!ane

Kevlar

=onomers > ben)ene(+,5(dicarbo!ylic acid and ben)ene(+,5(diamine

ses> clothing; bullet proof vest, firefighters, crash helmet


16/26

Hydrogen bonds form between polyamide chains from the H in H to the 8 inC*8


17/26

d) 'egrada"le &olymers Chemists have helped to minimise environmental waste by developing degradable

polymers similar to poly%lactic acid&. 'his can be used to make packaging, wastesacks, disposable eating utensils and internal stitches. ?ll of these products willdegrade over time


18/26

Aromatic reactions

3- Analysis*#romatogray

18


19/26

a) 6ntroduction Chromatography is an analytical techniquefor separating components in a

mi!ture.

=ethod> %'AC&+. Chromatography plate placed in the chromatography tank with the sample

line at the bottom.. 'he plate is in contact with solvent but important that the solvent level is

below the sample that has been placed.0. 'ank is covered and sample left until the solvent front has reached near the

top of the plate.5. 'he plate removed and the position of the solvent front immediately marked.3. 'he solvent allowed evaporating and plate is analysed.

Mery versatile and is able to separate components with very similar physicalproperties in a comple! mi!ture

Iorks on basis that different components have different affinity for stationary andmobile phase.

Buring chromatography a mobile phase sweeps mi!ture over stationary phase.

'he stationary phase interacts with component slowing them down. 'he greaterthe attraction the slower the components move.

"eparation;o solid adsorption

o liquid solubility

ses> drugs, plastics, flavouring, food and forensics.

") ;#in-layer *#romatograyo /n 'AC, the stationary phase is a thin layer of adsorbent solid silica, "i8, or

alumina, ?l80coated onto a plate which is glass or plastic 'AC plate.

o =obile phase is liquid solvent which moves vertically up the plate.o ? dry 'AC plate separates by adsorption, some bind more strongly than others.

o 'he end result id chromatograph.

o 'he compounds appear as spot on the plate if coloured can be easily seen but if

not than a locating agent necessary. ?lso M radiation used to show upcomponents by fluorescence.

o 7fvalues * distance travelled by sample@distance travelled by solvent

o 7fvalues differ from solvent to solvent

o Components of mi!ture identified by comparing with known reference samples or

using 7fvalues.o Aimitation>

+. "imilar compounds have similar 7f value.. Bifficult to find solvent that separates all compounds in the mi!ture. /f too

soluble in solvent wash away if not they wonDt move at all.

c) as *#romatogray, *

sed to separate volatile components in a mi!ture. seful for organic compoundswith low boiling points.

=obile phase is a carrier gas e.g. nitrogen %which is inert&

"ample mi!ture is inEected into machine

'he retention time is the characteristic time it takes for a particular substance topass through the system under set conditions

'he output is a graph of detector response %y(a!is& against retention time %!(a!is&.

'his provides a spectrum of peaks for a sample representing the compoundspresent in a sample eluting from the column at different times

19
http://en.wikipedia.org/wiki/Laboratory_techniqueshttp://en.wikipedia.org/wiki/Analytehttp://en.wikipedia.org/wiki/Laboratory_techniqueshttp://en.wikipedia.org/wiki/Analyte


20/26

Components of the mi!ture are identified by their retention times

'he use of retention times to identify substances is limited by the facts that similarcompounds often have similar retention times and that unknown compoundshave no reference retention times for comparison

9) *-MS

:as chromatography can be combined with mass spectrometry

Components separated by :C are directed into a mass spectrometer

? computer can analyse a componentDs mass spectrum or compare it with adatabase for positive identification

:C(=" is used in forensics, environmental analysis, airport security and spaceprobes

:C(=" is a far more powerful analytical technique than gas chromatography onits own

1) Pa&er *#romatogray

o "tationary phase is water adsorbed on paper

o =obile phase is liquid solvent

o "eparates by partition %relative solubility of solute between mobile and stationary

phases&o 7fvalues calculated from distance travelled by sample@distance travelled by

solvento 7fvalues differ from solvent to solvent

o Components of mi!ture identified by comparing with known reference samples or

using 7fvalueso /n two(way chromatography, the chromatogram is run in one solvent then rotated

through G-and run in a different solvento 'his is more effective than one(way chromatography because it is highly unlikely

that solvents have the same 7 fvalueso /n the separation of amino acids, the spots are invisible and have to be shown up

by adding ninhydrin or iodine %or by observing under u@v light&

S&ectrosco&y

1) 6nfrared s&ectrosco&y

'he bonds in organic molecules absorb certain frequencies of infrared radiation. 'he frequencies that are absorbed can be used to identify the presence of certain

bonds.

'he absorptions you need to know are>

a& C*8 +61- ( +3- cm(+

b& (8H in alcohols 00- ( 033- cm(+

c& (8H in carbo!ylic acids 3-- ( 00-- cm(+%broad peak&d& C(8 +--- ( +0-- cm(+

%0am&le 1 > &ro&anone

20


21/26

8f the absorptions mentioned above, only the C*8 peak will be present inpropanoneDs /7 spectrum>

%0am&le $ > et#anol

8f the absorptions mentioned above, the 8H in alcohols and the C(8 peaks will bepresent in ethanolDs /7 spectrum>

8(H C(8

21


22/26

%0am&le 3 > %t#anoic acid

8f the peaks mentioned above, the 8H in acids, the C*8 and the C(8 peaks will bepresent in ethanolDs /7 spectrum>

8(H C*8 C(8

$) Mass s&ectrometry

/n a mass spectrum for a compound, the peak furthest to the right %or highest m@)& iscalled the molecular ion peak and it gives the molar mass of the compound.

%0am&le > et#anol

'he molar mass of ethanol is 56. 'he molecular ion peak is indicated by the arrow.

3) MR s&ectrosco&y

a) 6ntroduction

o =7 stands for nuclear magnetic resonance

o /t can be used on the nuclei of atoms that contain an odd number of protons

and neutrons e.g. +0C and +Ho =7 involves the interaction of materials with low(energy radio waves

22


23/26

o =7 is the same technology as that use in magnetic resonance imaging

%=7/& to obtain diagnostic information about internal structures in bodyscanners

o /n =7, the position of each peak is measured from the peak produced by a

standard substance called tetramethylsilane, '=", which is %CH0&5"io "olvents for running =7 spectra must not contain H atoms so deuterated

solvents, such as CBCl0, are used

") Proton MR

'he peaks in a proton =7 spectrum give the following information about thehydrogen nuclei in an organic compound>

'he number of peaks shows the number of different types of proton

'he ratio of the areas under the peaks gives the number of protons of eachtype

'he distance along the a!is %chemical shift& shows the environment of eachtype of proton %see data sheet&

'he splitting pattern shows how many neighbouring protons there are B8 can be used to identify 8H groups

%0am&le 1 > %t#anoic acid

'wo different types of H so two peaks

"i)e of peaks in ratio 0>+ (C8CH0at .-(.G and C88H at ++.-(++.

o splitting as HDs not on adEacent atoms

Pea s&litting

+& ? peak will be split if there are HDs on the ne!t C or 8.& "plitting follows the n+D rule0& +H splits an adEacent peak into two %a doublet&, HDs split an adEacent peak

into three %a triplet& and 0HDs split an adEacent peak into four %a quartet&5& /f splitting occurs from both sides a more complicated pattern is produced %a

multiplet&

23


24/26

3& /n a doublet, the ratio of the areas under the peaks is +>+. /n a triplet, the ratioof the areas under the peaks is is +>>+. /n a quartet, the ratio of the areasunder the peaks is +>0>0>+

%0am&le $ > %t#anol

'hree different types of H so three peaks

"i)e of peaks in ratio 0>>+

7(CH0at -.(+.6, (8CH7 at 0.0(5.0 and 7(8H at +.-(3.3

CH0peak split into triplet by Hs on ne!t C

CHsplit into quartet by 0Hs on ne!t C

'he 8H peak is not split and does not split other peaks. 'echnically, this H is

decoupled by fast proton e!change

5se of '$ it# > and > grou&s

Beuterium is an isotope of hydrogen that does not produce an =7 peak

/f an alcohol, carbo!ylic acid or amine is shaken with B8, the H of the 8H or Hgroup is replaced by B and the peak disappears from the spectrum

") *ar"on-13 MR

'he peaks in a +0C =7 spectrum give the following information about the carbonnuclei in an organic compound>

'he number of peaks shows the number of different types of C

'he distance along the a!is %chemical shift& shows the environment of eachtype of C %see data sheet&

24


25/26

%0am&le 1 > %t#anol

'wo different types of C so two peaks

*(8 at 3-(- and *(* at 3(33

SourceC #tt&C??.c#emguide.co.u?analysis?nmr?inter&retc13.#tml

25


26/26

ocr chemistry a-level unit 3 (ring,polymer & analysis) revision notes

Documents