UNIT 4

Achiral

Doesn’t have a chiral centre

Chiral

Has a chiral centre

Chiral centre

Atom in a molecule bonded to 4 different atoms or groups

Enantiomer

Non superimposable mirror

image forms of each other that's rotate the plane of polarized light in opposite directions

Isomer

Say molecular formula different arrangement of atoms

Optical activity

Occurs in molecules that possess chiral centres

these molecules can rotate the plain and polarized light

Stereoisomerism

Occurs in isomers with the same molecular formula but different arrangements of atoms in space

Structural isomer

Compounds with the same molecular formula but different arrangements of atoms

Racemic mixture/ racemate

An equimolar mixture of both enantiomers that produce no overall rotation of plane polarized light

Stereoisomerism

E-Z isomer

shown in alkenes

E- same atoms opposite each other ( trans)

Z- next to each other (cis)

the pi bond restricts rotation about the double bond

If a compound contains 2 or more chiral centres

it is possible to have stereoisomers that are not Mirror image forms of each other [called diastereoisomers]

optical activity mechanism

Light consists of waves that are vibrating in all planes

if light is passed through a polarising filter, the light that emerges is vibrating in one plane only [called plane polarised light]

if a solution of an enantiomer is placed in a beam of plane polarised light, the beam is rotated

The extent to which the plane of polarised light is rotated depends on:

• Particular enantiomer

• concentration of the enantiomer In solution

• Length of the tube containing the solution through which the light passes

• frequency of the light source used and the temperature also needed to be considered

enantiomer may rotates clockwise or anticlockwise

equal amounts of each in solution produces no overall rotation since rotation effects cancel each other out

the mixture results from external compensation as the effect is caused by two different compounds.

The equimolar mixture is called a racemic mixture.

Ibuprofen

contains Kyle Centre and is sold as a racemic mixture

since biological systems usually respond to one in the required way rather than the other, there is a danger that the unwanted form could have serious side-effects

Separation of the two enantiomer is called resolution would be difficult and expensive

When ibuprofen is taken, one of the enantiomer is much more biologically active than the other, but the less active form is converted by an enzyme in the body into the other, more active enantiomer

Benzene

Relative mr- 78

C6H6

kekules suggestion on benzene

• 6 membered ring

• containing alternating single and double C-C bond

Why kekules suggestion was incorrect

addition reaction

• It should react as an alkene and easily undergo addition reactions (eg reaction with aqueous bromine where bromine is decolorized) However it doesn't react this way

• to explain this, he suggested benzene had two forms in which one form changed to the other so quickly that an approaching molecule would it have time to react by addition ( double a single bonds alternate)

Why kekules suggestion was incorrect

bond length

• if Benzene had two forms, they would be two different bond lengths between carbon atoms in the molecule

• X-ray crystallography showed each C-C bond was the same length at 0.140 nm

• this distance between C-C double bond at 0.135 nm and a carbon carbon single bond at 0.147 nm

Benzene Enthalpy

• When cyclohexane is hydrogenated the energy released is 120 kJ mol-1

• If benzene existed as Kekules structure and all three double bonds are hydrogenated than the entropy change would be through -360 kJmol-1

• but when benzene is fully hydrogenated, the entropy change is -208 kJmol-1

• 152 difference, (resonance energy) suggests benzene is more stable than kekules structure and suggests it is incorrect

Bonding in benzene and ethene

remaining

orbital

benzene is a planer molecule and the angles between three adjacent carbon atoms are 120°.

Ethan C2H4, Each carbon atom is bonded to 2 hydrogen atoms and a carbon atom by sigma bonds

remaining outer P electrons of each carbon atom overlap above and below the plane of a molecule giving a localised pi orbital

delocalised bonding of benzene

fourth outer shell electron of c

each carbon atom is bonded to 2 other carbon atoms and a hydrogen atom by sigma bonds

Fourth outer shell electron of C is in the 2P orbital above and below the plane of the C atoms. These orbitals overlap to give a delocalised electron structure above and below the plane of C atoms.

if benzene underwent addition reaction

process with disrupt the staple delocalised electron system and the resulting product would be less stable

needs higher temperatures and a nickel or platinum catalyst

will also react with chlorine in addition to give hexachlorocyclohexane C6H6CL6. radical reaction that needs bright sunlight to be effective

Mechanism of electrophilic substitution

ring of

benzene has a delocalised ring of electrons above and below the plane of the carbon atom. This area of high electron density makes it susceptible to attack by an electrophile.

electrophile

an electron deficient species that can accept a loan pair of electrons

Nitration

HNO3 + 2H2SO4 = NO2+ + H3O- + 2HSO4-

nitrobenzene is yellow liquid that is reduced to phenyl amine C6H5NH2 by using tin metal and hydrochloric acid

If the temperature of nitration exceeds 50°C, other branches of nitro groups are produced

Halogenation- Bromine

bromine and benzene do not react together unless a catalyst is used eg. Fe/FeBr3/AlBr3

Pi Electron System is not sufficiently nucleophilic to polarise the bromine In the presence of iron bromide, the colonisation of bromine molecule is more pronounced

Br-Br is polarised, delta + and - by the pi electron system

polarised by the eletron deficient iron in catalyst

Halogentaion bromine mechanism

The role of the iron bromide is catalytic encouraging greater polarisation of the Br-Br bond so that attack by the ring electrons can occur but being regenerated at the end of the reaction

chlorination of benzene

anhydrous AlCl3/ FeCl3 as the catalyst

in industry- used as a continuous process, so any polychlorination is kept to a minimum

chloro benzene- important industrial chemical

is nitrated and nitro- derivatives converted to 2-nitrophenol and 2-nitrophenylamine. Insectide DDT can be manufactured by the reaction of chlorobenzene with tricbloroethanl

DDT

• insecticide

• problems related to toxicity

• problem with bio accumulation, persistence in the environment and presence in the food chain

Friedel- crafts alkylation

method to produce new C-C bonds, giving alykl derivatives of benzene eg. Methylbenzene

anhydrous AlCl3 as a catylst, HCl as product

Friedel- crafts alkylation

mechanism

Produces new carbon carbon double bond, giving alkyl derivatives of benzene

anhydrous aluminium chloride as a catalyst

1-chlorohexane with aq NaOH on heating under reflux

gives hexan-1-ol

CH2(CH2)4CH2Cl + NaOH —> CH3(CH2)4CH2OH + NaCl

Mechanism:

Benzene tend to react by electrophillic substitution, has little tendency to react with nucleophiles, as these would be replelled by the stable Pi system of electrons

strength in C-Cl bonds in chlorobenzene and chloroalkane

the stronger and shorter bond betweenC-Cl in chlrobenzene results from a non bonding p electrons pair on chlorine overlapping with he ring pi system of electrons.

The resulting bond needs much more energy to be broken and is stronger than an aliphahtic C-Cl bond

Forcing conditions are therefore needed to produce phenol from chlorbenzene

producing phenol equation

this reaction is not an environmentally viable method of producing phenol commercially. In industry phenol Talk is generally made from 1-methyl ethylbenzene This method has the advantage of Propanone is a useful co product of the reaction.

problems with alkylation

introduction of an alkyl group onto the ring activates the ring towards alkylation

as a result the product may contain 1,2- and 1, 4-diethyl benzene. to reduce polyakylation, the halogen alkane is added slowly to the benzene and the catalyst

another problem is that a primary carbocation formed during the reaction may rearrange to a secondary

reaction of 1- chloropropane

with benzene gives mainly 1-methyl ethyl benzene rather than 1- propylbenzene as the organic product

acids chlorides with benzene - give a ketone

manufacture of phenyl ethene

EG ethanoyl chloride with benzene

important industrial process used Friedel crafts sty!e reaction

Nucleophiles

ions or compounds possessing a lone pair of electrons that can seek out a relatively positives site

donates a pair

Eg. -OH, -CN, NH2

Primary, secondary and teritary alcohols

P- Only one alkyl group attached to the carbon hydroxide group

S- 2

T- 3

Electronegativity

Ability of an atom in a chemical bond to polarise electron density towards itself

Difference in boiling points between alcohols and alkanes

set up between

• In alcohol there are hydrogen bonds set up between the slightly positive hydrogen and lone pairs on oxygen in other molecules

• in alkanes the only intermolecular forces are vander waals dispersion forces hydrogen bonds are much stronger and take more energy to separate alcohol molecules

Forming primary and secondary alcohols from halagenoalkanes

refluxing

• Carried out by refluxing together the halogenoalkane and an aqueous solution of an alkali (Na or KOH)

• during reflux the liquid is evaporated, cooled and condensed to continue the reaction

• separated at the end of the reaction by fractional distillation from diff boiling points

Forming a primary alcohol Eg. Butan-1-ol from 1-bromobutane

substitution

CH3CH2CH2CH2Br + NaOH → CH3CH2CH2CH2OH + NaBr

nucleophilic substitution where the hydroxide ion acts as the nucleophile and attacks the relatively positive carbon atom on the C-Br bond

secondary alcohol formation example

Rate of hydrolysis for alcohols from halogenoalkanes

C—I > C—Br > C—Cl

yield of primary alcohols is usually higher than the yield of secondary alcohols

when producing secondary alcohols a alkene can be formed as a side product

greater yields of alkenes are produced if high concentrations of the alkali are used or if an ethanolic solution of the alkali

Formation of alcohols from the reduction of a carbonyl compound

aldehydes and ketones

Both aldehydes and ketones can be reduced to primary and secondary alcohols by using an aqueous solution of sodium tetrahydridoboarate (NaBH4)

use [H] to represent a reducing agent

Reducing carboxylic acids

dissolved in

Sodium tetrahydridoboarate is not powerful enough to reduce carboxylic acids and the stronger reducing agent lithiumtetrahydridoaluminate (LiAlH4)

Dissolved in ethoxyethane

Reactions with the reducing agents

excess

Sodium tetrahydridor borate is a much safer reaction as an excess of lithium tetrahydridaluminate is difficult to dispose and the solvent ethoxyethane is very flammable

To obtain a good yield of alcohol

the organic solvent

An access of the reducing agent is used

the excess reducing agent can be removed by adding a dilute acid

the organic products needs to be separated from the aqueous mixture

carried out using a separating funnel if the alcohol produced is immisciple with water or separated by adding an organic solvent such as ethoxyethane by the technique of solvent extraction

Reaction of primary and secondary alcohols with hydrogen halides

Halogenoalkanes are produced by reacting the alcohols with hydrogen halides

the reactions are generally slow and reversible often giving poor yields

Chlorination

Passes hydrogen chloride gas into the alcohol in the presence of anhydrous zinc chloride, acting as a catalyst

Chlorination using phosphorus chloride (V)

Room temperature

one problem with this reaction is that phosphorus oxide trichloride (POCl3) is a liquid and needs to be removed from the reaction mixture. if the halogenoalkane has a similar boiling temperature to POCl3 then separation becomes difficult

Chlorination using sulphur (VI) oxide dichloride SOCl2

An advantage to this method is that the co products sulphur oxide and hydrogen chloride are both gaseous and are easily lost from the reaction mixture, ensuring easier separation

Bromination

‘in situ’ reaction: potassium bromide and 50% sulfuric (VI) acid is heated

the sulfuric acid protonates the alcohol and this then reacts with the bromide ions from the potassium bromide

CH3CH2CH2CH2OH + KBr + H2SO4 → CH3CH2CH2CH2Br + KHSO4 + H2O

Iodination

warm d

Warm dump red phosphorus and iodine together to form phosphorus iodide (III) PI3

2P + 3I2 → 2Pl3

3CH3CH2CH2OH + PI3→ 3CH3CH2CH2I + H3PO3

Reaction with carboxylic acids

Primary and secondary alcohols react with carboxylic acids to give esters

Alcohol + COOH → ester + H2O

the reaction is reversible and eventually the mixture will reach a position of equilibrium

To increase the yield of Esters

distilled

Little concentrated sulfuric (VI) acid is added to the mixture of the alcohol and the carboxylic acid, the mixture is heated under reflux

the products can then be distilled and the Ester collected at its boiling temperature

the distillate generally consists of the Ester and water together with a little unreacted alcohol and carboxylic acid. Many esters are immiscible with water and the distillate often consists of two layers

Separating funnel used to extract the Ester

dried

Then shaken with sodium hydrogen carbonate solution to remove any remaining carboxylic acid

Ester is then dried with anhydrous calcium chloride which reacts with any remaining alcohols

it can then be redistilled to give a pure product

Reactions with ethanoyl chloride

Alcohol reacts rapidly with ethanyol chloride giving an ester. During this reaction misty fumes of hydrogen chloride are seen.

Gives a better yield of an Ester than using a carboxylic acid as the reaction is not reversible

however the cost of acid chloride means this is not a cost efficient process in industry

Reactivity of phenols compared to alcohols

lone pair overlap

extended delo

Reactivity is very different

One of the lone pairs of oxygen atoms overlap with the delocalized Pi system to form a more extended delocalized system.

As a result the C—O bonds in phenols is shorter and stronger than in alcohols

making C—O bond fission in phenols harder than co-bond in alcohols

the extended delocalization creates a higher electron density in the Ring and makes the ring structure more susceptible to attack by electrophiles

Acidity of phenol

Phenol is a very weak acid and the position of the equilibrium lies to the left

phenol can lose a hydrogen ion because the phenoxide ion formed is stabilized to some extent. The negative charge on the oxygen atom is delocalized around the ring

The more stable the ion the more likely it is to form

Why is phenol only a weak acid

one of the lone

One of the lone pairs on the oxygen atom overlap with the delocalized electrons on the benzene ring

this overlap leads to a delocalization which extends from then ring out over the oxygen atom resulting in the negative charge no longer entirely localised on the oxygen, but is spread out around the whole ion

spreading the charge around makes the ion more stable

oxygen is the most electronegative element in the ion and the delocalized electron will be drawn towards it. This means that they will be still a lot of charge around the oxygen

which will tend to attract the hydrogen ions back again

Phenol with the sodium hydroxide

gives a

Gives a colourless solution containing sodium phenoxide- showing it must be acidic

hydrogen ion has been removed by the strongly basic hydroxide ion in the solution

this shows phenol reacting as an acid losing a proton to the aqueous hydroxide ion

The presence of substituents on the benzene ring affects the acidity of phenol

phenol tion with sodium carbonate/ sodium hydrogen carbonate

Phenol is not strong enough to react to produce carbon dioxide. No bubbles are produced

shows it's only a weak acid

Phenol with metallic sodium

slow

Slow reaction because phenol is such a weak acid

phenol is warmed in a dry tube until it is molten and a small piece of sodium is added there is some fizzing as hydrogen gases given off

mixture is left in the tube will contain sodium phenoxide

Directing effect of the -OH group

Has more activating effect on some positions around the ring than others

incoming groups will go into these positions much faster

has a 2,4,6 directing effect, incoming groups will tend to go into the 2, 4 or 6 position

Phenol with bromine

presence activate

Presence of an OH group bonded directly to a benzene ring will activate the ring to attack by electrophiles causing directing effect

when female reacts with bromine the increased electron density in the ring polarises the bromine molecules

Aqueous bromine reacts with phenol to produce a white precipitate of 2,4,6- tribromophenol

Colour change in the bromine test

Aqueous bromine= orange

products= colourless solution and a white precipitate

this means the orange colour disappears since the bromine is decolorized

How is reaction with bromine with phenol different to an alkene

In an alkene reaction, it will result in the decolorization of bromine without the additional formation of a white precipitate

mechanism for this reaction is electrophilic substitution. Electrophile is Br+

Bromine reaction with water

gives HOBr and HBr

Br2 + H2O = H-O-Br + HBr

concentration of Br+ from HOBr is very small as a position of the equilibrium lies well to the left

Reaction of Phenols and ethanoyl chloride

react as

Alcohols and phenols can react as nucleophiles by the use of their oxygen lone pairs

the delocalisation of an electron pair from the oxygen atom in a phenol means it is more difficult for a phenol to react as a nucleophile

as a result carboxylic acids are not suitable reagents to make an ester with phenol

Phenol and ethanoyl chloride

Slow at room temp

Phenol and a pyridine ( a base)

Can be added to speed up the reaction

the pyrdine react with the hydrogen chloride co-product to give pyridinium chloride (C5H5NH+Cl-)

Phenol and ethanoic anhydride

Ethanoyl chloride is an expensive reagent and ethanoic anhydride is used in preference

Phenol and acyl chloride

Less reactive

aqueous conditions can be used as the acyl chloride is only slowly hydrolysed by water

Test for phenols

Phenol will react with iron (lll) chloride to produce a purple color in the aqueous solution

the colour is produced by a complex being formed between the two reagents

group in aldehydes and ketones

Both have a polar C=O group

this occurs because oxygen in this bond has a greater electronegativity, leaving the carbon atom slightly electron deficient

the double bond comprises a ó bond and a pi bond above and below the plane of the sigma bond caused by p-p orbitals overlapping

Aldehyde

Will have at least one hydrogen atom

C-R-H=O

Ketone

Has a carbonyl carbon atom directly bonded to two other carbon atoms

Formation by the oxidation of primary alcohols

Primary alcohols are oxidized to aldehydes, then further to carboxylic acids

the oxidising agent is acidified potassium (or sodium) dichromate K2Cr2O7H+

colour change from orange to green

Formation by the oxidation of secondary alcohols

Under normal conditions a ketone cannot be further oxidized by this method

acidified potassium manganate can also be used as the oxidizing agent

purple/pink colour of the manganate ion is lost and a colourless solution containing aqueous manganese ions remain

Distinguishing between aldehydes and ketones

Tollens agent

Ammoniacal silver nitrate)

• made by adding aqueous sodium hydroxide to silver nitrate solution until a brown precipitates of silver (l) oxide is formed

• aqueous ammonia is then added until the silver oxide just redissolves

• aldehyde is then added to this reagent and the tube is gently warmed in a beaker of water

Result of Tollens reagent

• If the compound is an aldehyde, a silver mirror coats the inside of the tube as the Ag+ ions are reduced to Silver

• less soluble aldehydes are reluctant to react in this way

ketone doesn't form a silver mirror

Distinguishing between aldehydes and ketones

Fehling's reagent

tartate

Reacts to an aldehyde but not a ketone

also shows the presence of an aldehyde group in reducing sugars (eg. Glucose)

• Fehling A is an aqueous solution of copper (ll) sulfate

• Fehling B is an aqueous solution of potassium sodium tartrate (made alkaline with sodium hydroxide)

• on mixing the two Solutions a deep blue solution containing complex copper (ll) ion is produced

Result of Fehlings solution

If an aldehyde is added to the deep blue solution and mixture is warmed in a water bath, then the aldehyde reduces the complex copper (ll) ions and the deep blue solution is replaced with an orange red precipitate of copper (l) oxide

Decomposition of Fehlings solution

Solution decomposes easily and the two solutions are mixed when needed.

Benedict's reagent is a similar reagent containing a complex copper ll ion and is similarly reduced to Copper (l) oxide

Advantage to Benedict's reagent

• Stable and is safer to use as it is less alkaline

• the test using the Solutions is effective in identifying ALIPHATIC aldehydes but doesn't react with aromatic aldehydes

The reduction of aldehydes and ketones

Aldehydes and ketones are reduced to primary and secondary alcohols respectively

reducing agents are sodium tetrahydridoborate (NaBH4) or lithiumtetrahydridoaluminate (LiAlH4)

NaBH4 is preferred as it is safer and can be used in acquies conditions

Nucleophilic addition reactions of aldehydes and ketones

Both contain a polar carbonyl bond C=O

the relatively electron of deficient carbon atom can be attacked by a nucleophile (Eg. CN)

Identifying aldehydes and ketones using

2,4-dinitrophenylhydrazine

2,4 DNP is used because a react with an aldehyde or ketone to produce an orange/yellow or red crystalline solid which can be filtered off and purified

it's it's dissolved in an acid to give a solution (Brady's reagent)

Melton temperature of purified product

( 2,4 dinitrophenylhydrazone) is taken and compared with a table of known values to identify the aldehyde or ketone present

called “making a derivative/ derivatisation”

Mechanism of this reaction

Takes place by nucleophilic addition-elimination (sometimes called condensation reaction)

this happens because the nitrogen atom has a lone pair of electrons attached to it, and so act as a nucleophile, attacking the slightly positive carbon atom of the C=O in the aldehyde or ketone

followed by the elimination of a water molecule

derivatives are called hydrazones

Iodoform reaction/ triiodomethane

Triiodomethane CHI3 is a yellow solid that has some uses as an antiseptic

The yellow solid formed helps to identify a methyl carbonyl group (CH3CO)

A positive result from the iodoform

• Methanal would be the only aldehyde to give a positive result

• methyl ketones would be the other type of compound to give a positive result

Contents and reaction of iodoform

The organic compound is warmed with a color solution of iodine in aqueous sodium hydroxide (alkaline iodine).

or aqueous mixture of potassium iodide and sodium chlorate (l), NaCOl

Test can also be used on methyl alcohols

The OH group in alcohols must be attached to a carbon atom with a hydrogen and a methyl group attached

therefore this will work for ethanol and secondary alcohols

Acidity of carboxylic acids

extent of ionisation

They are weak acids and the extent of the ionization in aqueous Solutions is very small

they are acidic because of the hydrogen in the COOH group

carboxylic acids > phenols > water/alcohols

Test for relative difference in acidity

Can be shown by their reaction with sodium hydrogen carbonate solution

only carboxylic acids are strong enough to produce colorless bubbles of carbon dioxide gas