rings, acids and amines[1]

8/14/2019 Rings, Acids and Amines[1]

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Any compound with the benzene ring is called an aromatic compound

Contains one or more benzene rings-

Arenes are aromatic hydrocarbons

Each one is bonded to one hydrogen atom and has the molecular formula C6H6-

A benzene molecule is a ring of 6 carbon atoms

Aromatic compounds14 June 2012 10:55

Rings_ polymers and analysis Page 1


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Each of these carbon atoms are bonded to one hydrogen atom-

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A benzene molecule is a ring of six carbon atoms and the molecular formula of this is C 6H6

They occur naturally in materials like crude oil and coal-

They can also be artificially produced in refineries from crude oil-

Arenes are aromatic hydrocarbons that contain at least one benzene ring

They are named a bit like hydrocarbons. For example, if a chlorine atom replaces one of the hydrogens, it is called

chlorobenzene. If C2H5 is attached, then it is called Ethylbenzene

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If more than one hydrogen is replaced, then the rules are still similar to hydrocarbons. For example, if CH3 replaces the

hydrogen of carbon 1 (The one top carbon) and a bromine atom replaces the hydrogen on carbon two (the carbon to the right

of carbon 1), then it is called 2-bromomethylbenzene

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Benzene derivatives are formed when a hydrogen on a benzene is replaced by an atom or a group of atoms. These can be named

using the following rules:

Benzene can also be used as a starting point for the synthesis of other aromatic materials, like phenol and styrene

Kekul came up with a model for benzene which suggested that the bonds between the carbons are constantly changing between

double and single bonds. This model was, however, disproved later on when more tests with benzene were conducted, showingthat Kekul's benzene has more energy than benzene actually does.

Benzene is a cyclic hydrocarbon that has six carbon atoms and six hydrogen atom-

The six carbon atoms are arranged in a planar hexagonal ring, with each carbon bonded to two other carbons and one

hydrogen

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The shape around each carbon atom is trigonal planar with a bond angle of 120O-

Each carbon atom has four outer electron shells, three of which bond to two other carbon atoms and one hydrogen atom.

These are all -bonds. The fourth outer electron shell is left in a 2p orbital above and below the plane of the carbon atoms

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The fourth electron of the carbon atom overlaps with the electrons in other p orbitals of other carbons. This causes a ring of

electron density above and below the plane of carbon atoms

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This overlap produces a system of -bonds which spread over all six of the carbon atoms, meaning that the p electrons become

spread over the whole ring and are described as delocalised

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The 6 delocalised electrons are spread over the 6 carbon atoms in a cloud, as shown below:-

The six carbon-carbon bond lengths in benzene all have the same wave length of 0.139 nm, which is in between the wave

lengths for single and double wave lengths between carbons

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The delocalised model of benzene was developed and accounted for the problems in Kekul's model. It has the following features:

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Decolourise bromine water-

React with strong acids such as HCl-

React with the halogens chlorine, bromine or iodine-

This means that, under normal conditions, benzene will not:-

An addition reaction would result in a product a lot less stable than benzene due to electrons from the delocalised system

needing to bond to the atom/group of atoms being added, so therefore the reaction would not be energetically favourable and

disrupt the delocalisation of the ring structure

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The strong stability of benzene causes it to be less likely to participate in addition reactions

These occur due to the delocalised ring of electrons-

The product formed retains the delocalisation of the electrons, and therefore also keeps the stability of the benzene ring-

Substitution reactions are the most common types of reaction that benzene participates in-

Benzene and its derivatives typically take part in substitution reactions

Benzene15 June 2012 13:27



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One H-atom is replaced by a nitro (NO2) group-

The benzene reacts with a mixture of concentrated nitric and sulphuric (to act as a catalyst) acid at 50 OC-

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The equation for this is C6H6 + HNO3 C6H5NO2 + H2O-

The nitrating mixture is prepared in a flask (round bottom/pear shaped)-

The nitric and sulphuric acid are mixed carefully while cooling in a mixture of cold water-

Benzene is then added to the nitrating mixture at a temperature below 50OC-

Once added a reflux condenser is fitted to the flask and the mixture is heated to 50OC in a water bath-

An example of this is the nitration of benzene-

Some halogen carriers are: FeCl3, FeBr3, AlCl3, AlBr3 and iron metal-

Benzene doesn't react with halogens on their own, however they can react with them in the presence of a halogen carrier

(a type of catalyst

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C6H6 + h2 C6H5h + Hh-

In this reaction, benzene reacts with a halogen (h) in the presence of Feh3 or Alh3 etc-

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2 delocalised electrons donated to electrophile, covalent bond formed C-H bond breaks, 2 electrons returned to

delocalised ring Products formed

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The general mechanism of electrophilic substitution of benzene-

Another example is the halogenation of benzene-

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The diagram below explains why Br2 reacts with cyclohexene:

This is because benzene does not have the high electron density that cyclohexene does so the Br2 molecular does not become

polar

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The iron fillings (or any halogen carrier) generates a more powerful Br+ electrophile so that the reaction can now take place-

When bromine water is added to benzene, no reaction takes place, unless some iron fillings are added

Delocalised electrons are shared between more than two atoms

An electrophile is an atom (or group of atoms) that is attracted to an electron-rich centre, where it accepts a pair of electrons to

form a new covalent bond

Electrophilic substitution is a type of substitution reaction in which an electrophile is attracted to an electron-rich centre or atom,

where it accepts a pair of electrons to form a new covalent bond

A reaction mechanism is a series of steps that, together, make up the overall reaction

A curly arrow is a symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of

covalent bonds



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Phenols are a class of organic compounds in which a hydroxyl group, -OH, is attached directly to a benzene ring

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Due to strong intermolecular bonds-

A pink solid (when pure)-

An aromatic compound-

Solid at room temperature and pressure-

The benzene ring stops it from being more soluble-

Slightly soluble in water-

It is:-

C6H5OH + aq C6H5O- + H+-

When dissolved in water and weak acid solution is formed-

C6H5OH + NaOH C6H5O-Na+ + H2O-

When neutralised with NaOH a salt (sodium phenoxide) and water are made-

2C6H5OH + 2Na 2C6H5O-Na+ + H2-

When reacted with a reactive metal (e.g. sodium), hydrogen gas and a salt is formed (which is soluble in water)-

The above reactions of phenol occur due to the breaking of the O-H bond (as if it were a carboxylic acid)-

This is due to the OH bond-

Phenol is a lot more reactive than benzene-

The simplest of the phenols is called phenol:

(2,4,6 tribromophenol)

C6H5OH (aq) + 3Br2 (aq) COHCBrCHCBrCHCBr + 3HBr-

This reaction can be used as test for phenol-

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This also causes the O- to become less negatively charged, making it more difficult to accept protons and is therefore less

basic than HO--

The lone pair on the oxygen atom is drawn into the benzene ring creating a more dense electron region which can then

polarise the bromine molecule to make it become an electrophile so that a reaction can take place

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Phenol can undergo electrophilic substitution reactions with Br without the need for a halogen carrier

Antiseptics-

Found in surfactants and detergents-

Disinfectants-

Plastics-

Polymers-

Pharmaceuticals-

Epoxy resins for paints-

Uses of phenols

Phenols29 June 2012 16:18



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H2O + H+ ----> H3O+-

As a base:-

H2O ----> HO- +H+-

As an acid:-

H2O + H2O ----> H3O+ + OH--Overall:

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Water can act as a base or an acid

ROH + H+ ----> RO2+-

As a base:-

ROH ----> RO- + H+-

As an acid:-

ROH + ROH ----> RO2+ + RO--

Overall:-

When an alcohol acts as a base it cleaves at the RO bond and when it acts as an acid it breaks at the OH bond-

CH3CH2O- is more basic than just HO- as there is a positive inductive effect which increases the negativity of the O- which

allows it to accept protons easier

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Alcohols can also act as a base or an acid

Phenol

Amphoteric nature of hydroxyl compounds18 September 2012 14:27



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Carboxylic acids with between one and four carbons are very soluble in water due to the highly polar C=O and O -H bonds allowing

the carboxylic acid molecules to form H-bonds with the water molecules

As the number of carbon atoms increases, solubility decreases, which is due to the longer non-polar hydrocarbon chain in the

molecule

Carboxylic acids are weak acids when compared with nitric, sulphuric and hydrochloric acids

The salts that are formed are called carboxylates and they are named by using the first part of the carboxylic acid name

and adding -oate on the end, e.g. Methanoic acid forms methanoate

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Reacting with metals, carbonates and bases to form salts-They participate in the typical acid reactions

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In acid reactions of carboxylic acids, the product is a carboxylate and contains the carboxylate ion and is shown as:

A carboxylic acid reacts with metals to form a salt and hydrogen gas

A carboxylic acid reacts with carbonates to form a salt, carbon dioxide and water

A carboxylic acid reacts with aqueous bases to form a salt and water

Carboxylic acids22 June 2012 14:25



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The bond in the carbonyl (C=O) is attracted slightly to the oxygen atom due to it being more electronegative-

The carbonyl functional group is shown below:

Aromatic aldehydes and ketones are compounds with a benzene ring and a carbonyl group

The functional group is the part of an organic molecule responsible for its chemical reactions

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Carbonyl compounds29 June 2012 14:23



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A solution of 2,4-DNP in a mixture of methanol and sulfuric acid is known as Brady's reagent-

This precipitate, called a 2,4-dinitrophenylhydrazone derivative, confirm the presence of the carbonyl functional group,

C=O in an organic compound

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When Brady's reagent is added to an aldehyde or ketone, a yellow or orange precipitate is formed-

Aldehydes and ketones can be detected using the reagent 2,4-dinitrophenylhydrazine (2,4-DNP or 2,4-DNPH)

Aldehydes are easily oxidised by Tollen's reagent and ketones are not oxidised at all-Once a compound has been identified as a carbonyl using 2,4-DNPH it can be further classified

Adding NaOH (aq) to Ag(NO)3 (aq) until a brown precipitate is formed-

Dilute aqueous ammonia is then added until the precipitate just dissolves-

Tollen's reagent can be made by:

In Tollen's reagent the oxidising species is the aqueous silver(I) ion, Ag+ (aq)

A silver precipitate forms-

Propanal-

A silver precipitate forms-

Ethanal-

No observation-

Propanone-

No observation-

Butanone-

Observations when Tollens is added to

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General formula for the oxidisation of an aldehyde using Tollens reagent:

The yellow/orange solid 2,4-DNPH derivative is slightly impure-

The impure product is filtered to produce purified yellow/orange crystals which are allowed to dry-

Different types of ketones have different melting points-

The melting point of purified derivative is measured and recorded-

A carbonyl compound can be identified by using the following:

Chemical tests on carbonyl compounds25 June 2012 09:17



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Therefore, because a primary alcohol forms an aldehyde when oxidised, an aldehyde will form a primary alcohol when

reduced. Same with a ketone and secondary alcohols

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Reduction is basically the opposite of oxidisation

A reducing agent is shown as [H] and normally in an equation two of these are used

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A nucleophile is an atom or group of atoms attracted to an electron-deficient centre, where it donates a pair of electrons

to form a new covalent bond

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A curly arrow is a symbol used in reaction mechanisms to show the movement of an electron pair in the breaking orformation of a covalent bond

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The mechanism for the reduction of propanal

Reactions of aldehydes and ketones29 June 2012 14:34



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Propanoic acid + Ethanol Ethyl Propanoate + Water-

An ester can be made by reacting a carboxylic acid with an alcohol in the presence of an acid catalyst

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Ethanoic anhydride + Methanol Methyl ethanoate + Ethanoic acid-

Due to not be reversible-

Gives a much better percentage yield than using carboxylic acids-

An ester can be made by heating gently an acid anhydride with an alcohol

Called acid hydrolysis-

Propyl ethanoate + Water Ethanoic acid + Propan-1-ol-

An ester can be hydrolysed by heating under reflux with dilute sulphuric/hydrochloric acid. Water breaks down the ester, with the

acid being used as a catalyst

Sometimes called saponification- Ethyl propanoate + Sodium hydroxide Sodium propanoate + Ethanol-

An ester can be hydrolysed by heating under reflux with an aqueous alkaline

Alkyl-alcohol-oate-

Naming

Esterification is the reaction of an alcohol with a carboxylic acid to produce an ester and water

Hydrolysis is a reaction with water or hydroxide ions that breaks a chemical compound into two compounds

Esters30 June 2012 11:47



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This increases the points of contact and intermolecular forces which increases viscosity and reduces volatility-

Because of this they are relatively dense and solid at room temperature-

Saturated fatty acids tend to form straight-chain molecules that can pack tightly together

This means that there is less points of contact and is therefore a liquid at room temperature-

Unsaturated fatty acids can't stack together as easily

Triglycerides occur naturally in animal and vegetable fats

Glycerol is an alcohol with three OH groups-

Where = Number of carbon atoms-

Where = Number of double bonds-

e.g. Octadeca-9,12-dienoic acid would be 18:2 (9,12)-

Where = The position(s) of the double bonds-

A shorthand notation for fatty acids is-

Fatty acids are long chained carboxylic acids-

They are triesters of glycerol and three fatty acid molecules

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Below is an example of the formation of a simple triglyceride

Cis-fats present little danger to health, but trans-fats are thought to increase the risk of coronary heart disease

Unsaturated fatty acids naturally exist in the cis form

Trans fats have a higher melting point that cis fatsThe food industry often removes some of the double bonds by partial hydrogenation to make them solid a room

temperature, which has the side effect of many of the structures to change from cis to trans

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A cis-unsaturated fatty acid:

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A trans-unsaturated fatty acid:

Good for health-

More dense-

Higher percentage of proteins compared to LDL-

Made when triglycerides from unsaturated fats combine with cholesterol and proteins-

Lowers blood cholesterol levels-

Helps to remove fatty plaques and atherosclerosis-

Transports cholesterol from body tissues to liver where it is broken down-

High Density lipoproteins

Bad for health-

Main cholesterol carrier in blood-

Made when triglycerides from saturated fats combine with cholesterol and proteins-

Bind to receptors on cell membranes-

Taken up by the cells-

Circulate bloodstream-

Excess LDLs in diet overload membrane receptors, causing high blood cholesterol levels which can cause atheroma-

Low density lipoproteins

Biodiesel can be produced by using cooking oil, however natural sources of oil also occurs in crops

Triglycerides01 July 2012 11:32



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Where R = fatty acid carbon chain-

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Triglycerides in fats and oils are reacted with methanol or ethanol in the presence of a sodium or potassium

hydroxide catalyst

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Biodiesel is made by transesterification-



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Happens when the diazonium salt is reacted with an aromatic compound (such as a phenol or amine) under alkaline conditions-

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The product formed can be used an azo dye-

In the reaction, two benzene rings link together through an azo functional group-

Coupling reactions-

rings, acids and amines[1]

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