Introduction to Organic Chemistry - Here it is



Introduction to Organic Chemistry

All compounds that you will come across can be separated into TWO groups:

1. Inorganic compounds

2. Organic compounds

Inorganic compounds: these compounds do contain carbon (exceptions being carbonates and oxides of carbon).

Organic compounds: these compounds contain carbon and usually hydrogen and many contain oxygen and/or other elements.

Carbon and covalent bonds

Remember that carbon has four valency shell electrons and normally form covalent bonds

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Therefore all carbons will have four bonds

Types of carbon to carbon bonds

Formulae of organic compounds

o The molecular formula: this shows the actual number of atoms of each element in one molecule of the compound.

o The empirical formula: this shows the simplest ratio of atoms of each element present in the compound.

o The molecular formula: this shows the actual number of atoms of each element in one molecule of the compound.

o The full structural formula: this shows, in two-dimensional diagrammatic form, how the atoms are arranged in one molecule.

o The shortened structural formula: this shows the sequence and arrangement of atoms in one molecule in such a way that the nature and position of attachment of each functional group is shown without actually drawing the molecule

Questions

1. What is the empirical formula of the following i) C12H24 ii) C3H6 iii) C4H8O2 iv) CH4

2. Represent the following as shortened structural formula.

3. Represent the following as full structural formula

Functional Groups

Each organic compound is made up of two parts

• The hydrocarbon part composed of carbon and hydrogen atoms.

• The functional group or groups comprising another atom or group of atoms. The reactions of the functional group(s) determine the chemical properties of the compound.

Homologous Series

Organic compounds are classified into distinct groups called homologous series. Homologous series have the following characteristics

• All members contain the same functional group.

• All members have the same general formula.

• All members show similar chemical properties. Reactivity decreases as number of carbon atoms per molecule (molecular mass) increases.

• Physical properties show gradation along a series. As the number of carbon atoms per molecule increases, melting point, boiling point and density increase, solubility in water decreases.

• Each member differs in molecular formula from the next by CH2 or in relative molecular mass by 14.

• All members may be prepared by similar methods.

Naming Organic compound

First find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound

|No. of carbon atoms |Name prefix |

|1 |Meth- |

|2 |Eth- |

|3 |Prop- |

|4 |But- |

|5 |Pent- |

|6 |Hex- |

The second part is related to the functional group

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Isomerism

Isomerism is the occurrence of two or more organic compounds with the same molecular formula but different structural formulae. The compounds are called isomers.

Physical differences between isomers

The various isomers will have slightly different physical properties because they will experience slightly different intermolecular forces. Branch chains have weaker attractions than straight ones. Intermolecular forces are only effective over very short distances. The more branching there is in a chain, the more difficult it is for the molecules to get close to each other.

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Draw 3 isomers of C6H14( remember putting a methyl (CH3-) at the end of the chain lengthens the chain and does not count as branching)

Separating Crude oil (Fractional distillation)

The crude oil is heated and passed into a fractionating column which is cooler at the top and hotter at the bottom. The crude oil is split into various fractions. How far up the column a particular hydrocarbon gets depends on its boiling point. Suppose a hydrocarbon boils at 120°C. At the bottom of the column, the temperature is much higher than 120°C and so the hydrocarbon remains as a gas. As it travels up through the column, the temperature gets lower. When the temperature falls to 120°C, that hydrocarbon will start to turn to a liquid. It condenses and can be tapped off.

The hydrocarbons in the petroleum gases have boiling points which are so low that the temperature of the column never falls low enough for them to condense to liquids.

The temperature of the column isn't hot enough to boil the large hydrocarbons found in the fuel oil and this remains as a liquid. Some of the fuel oil is fractionally distilled under reduced pressure. The residue at the end of all this is bitumen, which is used in road making.

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Uses of the fractions

Petroleum gases

. Petroleum gases are a mixture of methane, ethane, propane and butane which can be separated into individual gases if required. These gases are commonly used as LPG (liquefied petroleum gas) for domestic heating and cooking.

Gasoline (petrol)

As with all the other fractions, petrol is a mixture of hydrocarbons with similar boiling points. Its use is fairly obvious!

Naphtha

Naphtha is used as a source of organic chemicals for industry as well as a constituent of petrol. Useful molecules like ethene and propene can be made by cracking the naphtha. You will find more about later

Kerosine

Kerosine is used as fuel for jet aircraft, as domestic heating oil and as 'paraffin' for small heaters and lamps.

Gas oil (diesel oil)

This is used for buses, lorries, some cars, and railway engines where the line hasn't been electrified. Some is also cracked to make other organic chemicals and produce more petrol.

Fuel oil

This is used for ships' boilers and for industrial heating. Some of the fuel oil is also distilled again, under reduced pressure, to make lubricating oil, grease, wax (for candles) and bitumen.

Bitumen

Bitumen is a thick black material which is melted and mixed with rock chippings to make the top surfaces of roads.

Cracking

Cracking involves breaking up larger hydrocarbon molecules into smaller ones. It is important since distillation of crude oil produces an excess of the larger hydrocarbons and insufficient quantities of the smaller, more useful, ones to meet modern demands.

• Thermal cracking uses heat.

• Catalytic cracking uses heat plus a catalyst.

Cracking always results in the formation of at least one alkene and is, therefore, a major source of alkenes, these smaller hydrocarbons, especially the alkenes, form the foundation of the petrochemical industry from which thousands of other compounds are manufactured.

Alkanes

The general formula for the alkanes is CnH2n+2. The alkanes are a family of simple hydrocarbons. They contain carbon-carbon single bonds. Alkanes are described as saturated hydrocarbons in the sense that they contain the maximum possible number of hydrogen atoms for a given number of carbons.

Physical Properties

As the molecules get bigger, the intermolecular forces between them increase. This means that more energy has to be put in to break the attractions between one molecule and its neighbours. At room temperature, C1 to C4 are gases, C5 to C17 are liquids and C18 onward are solids.

Reactions of alkanes

Because alkanes are saturated, they are relatively unreactive:

1. Alkanes burn in air or oxygen, alkanes burn with clean, blue, non-smoky flames forming carbon dioxide, water and heat

e.g. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

2. Alkanes undergo substitution reactions in substitution reactions, hydrogen atoms in alkane molecules are replaced by other atoms, e.g. halogen atoms.

Example: methane + chlorine

No reaction occurs in the dark; in bright light the reaction is explosive; in diffused light substitution occurs in stages, one chlorine atom at a time:

Uses

Alkenes

The general formula for the alkenes is CnH2n. The alkenes are another family (homologous series) of hydrocarbons. They all contain a carbon-carbon double bond. Alkenes are unsaturated hydrocarbons. The presence of the double bond means that they don't contain as many hydrogen atoms as the corresponding alkane.

Reactions of alkenes

Alkenes are reactive because one bond of the double bond is weaker and is easily broken:

1. Alkenes bum in air or oxygen. Alkenes bum with smoky yellow flames which contain unburnt carbon due to the high proportion of carbon in alkene molecules. Carbon dioxide, water and heat are formed.

e.g

2. Alkenes undergo addition reactions

In addition reactions, two molecules react to form one molecule:

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Coding for branched chains alkane

|SIDE CHAIN |CODED |

|CH3- |Methyl |

|CH3CH2 or C2H5- |ethyl |

Number the longest chain beginning with the end of the chain nearest a substituent.

Alkenes naming

You number from the end which produces the smaller numbers in the name.

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The chain is numbered starting form the end closest to the double bond, and the double bond is given the lowest number of its two double-bonded carbon atoms

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Name the following

Alcohols

The general formula for the alkenes is CnH2n+1OH. Alcohols all contain –OH group covalently bonded onto a carbon chain.

The production of alcohol (ethanol- C2H5OH)

Making ethanol by fermentation

Yeast is added to a sugar solution and left in the warm for several days in the absence of air ('anaerobic' conditions). Enzymes in the yeast convert the sugar into, ethanol and carbon dioxide. The process is known as fermentation. The biochemistry is very complicated. First, the sugar (sucrose) is split into two smaller sugars, glucose anti fructose. Glucose and fructose have the same molecular formulae, but different structures. They are isomers. Enzymes in the yeast convert these sugars into ethanol and water in a multitude of small steps. All we normally write are the overall equations for the reactions.

C12H22O11(aq) + H2O(l) → C6H12O6(aq) + C6H12O6(aq)

Sucrose glucose fructose

C6H12O6(aq) → 2C2H5OH(aq) + 2C02(aq)

Glucose ethanol

Yeast is killed by more than about 15% of alcohol in the mixture, and so it is impossible to make pure alcohol by fermentation. The alcohol is purified by fractional distillation. This takes advantage of the difference in boiling point between ethanol and water. Water boils at l00°C whereas ethanol boils at 78°C.

The liquid distilling over at 78°e is 96% pure ethanol. The rest is water. It is impossible to remove this last 4% of water by simple distillation.

Making ethanol by the hydration of ethane

Ethanol is also made by reacting ethane with water – a process known as hydration.

CH2=CH2(g) + H2O(g) → CH3CH2OH(g)

Uses

Ethanol is sold as 'industrial methylated spirit'. This is ethanol with a small amount of another alcohol, methanol, added to it. Methanol is poisonous, and makes the industrial methylated spirit unfit to drink, and so avoids the high taxes on alcoholic drinks.

Ethanol is widely used as a solvent - for example, for cosmetics and perfumes.

It is relatively safe and is a good solvent for the complex organic molecules Which don't dissolve in water.

Ethanol is also a useful fuel. It burns to form carbon dioxide and water, producing about two-thirds as much energy per litre as petrol. Mixtures of petrol with 10--20% ethanol- known as gasohol - are increasingly used in some countries, such as Brazil. These are countries which have little or no oil industry to produce their own petrol. On the other hand, they often have a climate which is good for growing sugar cane. Ethanol can be produced by fermenting the sugar, and then mixed with imported petrol. This saves money on imports.

Alcohol is also found in alcoholic beverages

Naming alcohols

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Carboxylic Acids

The general formula for carboxylic acids is CnH2n+1COOH.

Uses

Vinegar is a dilute solution of ethanoic acid. The familiar smell of vinegar is due to the acid. Naturally produced vinegars will have their origin described on the label- for example, 'wine vinegar' or 'cider vinegar'. Cheap vinegar may well be a product of the chemical industry and now has to be called 'non-brewed condiment'. Vinegar is used as a flavouring and a preservative. Ehanoic acid is also used in the production of acetate rayon- an artifical fibre made from the cellulose from wood pulp.

Laboratory preparation of ethanoic acid

This is done by the oxidation of ethanol. Ethanol is mixed with acidified potassium dichromate (VI) solution and heated under reflux. Oxidation occurs in two stages, first to ethanal(aldehyde) and then to ethanoic acid

C2H5OH + [O] → CH3HO(aq) + H2O(g)

Ethanol ethanal

CH3HO(aq) + [O] → CH3COOH(aq)

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Reactions with metals

A salt and hydrogen are produced

e.g. 2CH3COOH(aq) +Mg(s) → (CH3COO)2Mg(aq) + H2(g)

magnesium ethanoate

Reactions with carbonate and hydrogencarbonates

A salt, water and carbon dioxide are produces

e.g. 2CH3COOH(aq) + Na2CO3(aq) → 2CH3COONa(aq) + H2O(l) + CO2(g)

sodium ethanoate

Reaction of aqueous ethanoic acid

Ethanoic acid is partially ionized in water, therefore it is a weak acid which reacts in the same way as other acids.

Reactions of anhydrous ethanoic acid

They burn in air or oxygen to form carbon dioxide, water and heat

Aqueos ethanoic acid reacts with oxides and hydroxides of metals to produce salt and water

Reactions between carboxylic acids and alcohols

Heating a mixture of ethanoic acid. and ethanol with a few drops of concentrated sulphuric acid produces a sweet smelling liquid called ethyl ethanoate. This is one member of a family (homologous series) of compounds .called esters.

Esters

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Saponification of esters

Saponification is the name used to describe soap making. A mixture of natural oils and fats is boiled with an alkali. There are two products – soap and glycerol.

|oil/fat |+ |sodium |→ |sodium salt |+ |glycerol |

|(ester as in | |hydroxide | |of the acid | |(an alcohol) |

|palm oil) | |(alkali) | |(soap) | | |

| | | | | | | |

This reaction is an alkaline hydrolysis. The aqueous alkali splits up the ester to give an acid plus an alcohol. Since there is an alkali present, the dd formed immediately reacts with it, giving a salt - soap. Ordinary soap is made by saponification of the glyceryl esters of oleic acid, as in olive oil, stearic acid or palmiti!;. acid derived from palm oil The salt,

sodium stearate, is a common soap made from stearic acid.

Polymers

Polymers are macromolecules formed by linking together thousands of small molecules called monomers, usually in chains. Polymers are formed by polymerisation:

1. Addition polymerisation occurs when unsaturated monomers are linked to form a saturated polymer. The polymer is the only product and it has the same empirical formula as the monomer.

2. Condensation polymerisation occurs when monomers join with the elimination of a small molecule, e.g. water, from between each unit

The properties of polymers depend on:

• The type of monomer(s) from which it is formed

• The type of linkage between monomers.

Polymers may be man-made (synthetic) or occur naturally. Most synthetic polymers are referred to as plastics.

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|Advantages |Disadvantages |

|Easily shaped and moulded |Non-biodegradable-contribute to land pollution, |

|Inexpensive ,Light in weight, Easily coloured, |Produce dense smoke and, poisonous gases when burnt |

|Durable - do not rust, corrode or decay, |-contribute to air pollution, |

|Good thermal and electrical insulators, |Many are flammable – pose fire hazards |

|Can be made flexible or rigid, Some are very strong |Difficult to re-cycle |

| | |

| | |

| | |

Hydrolysis of polysaccharides and proteins

1. In the laboratory:Polysaccharides and proteins can be hydrolysed to monosaccharides and amino acids, respectively, by boiling with dilute acid.

2. In biological systems: Hydrolysis is achieved by enzymes during digestion.

3.

The structure of a protein can be determined by hydrolysing the protein and identifying the amino acids present by chromatography

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Sucrose(a dimmer)

Sucrose is familiar everyday sugar. It consists of a glucose unit and a fructose -unit-joined together to make a disaccharide. A disaccharide Is a-carbohydrate made from two simple sugars.

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A reaction in which two molecules join together with the loss of a small molecules (in this case, water) is known as a condensation reaction. You can reverse this reaction, and split sucrose up into glucose and fructose by reacting it with water. This is known as hydrolysis. The sucrose can be boiled with a dilute acid like dilute sulphuric acid. The acid acts as a catalyst - it is the water in the acid which actually reacts. Alternatively, it can be split into the simpler sugars by the enzyme, invertase. Invertase is also known as 'sucrase'.

Starch

Starch makes up the reserve energy supply for plants, and is present in, common foods like potatoes, wheat, maize and rice. Starch is a polysaccharide. It consists of long chains of glucose units joined together in the same way that glucose and fructose are joined in sucrose. There are two forms of starch. Soluble starch consists of chains of about 200 glucose units. Insoluble starch is made of heavily branched chains containing more than a thousand units.

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This is an example of polymerisation - joining up lots of small molecules to make a big one. Because of the loss of the water molecule every time a link is made between two glucose units, this is known as condensation polymerisation.

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