Organic Chemistry Notes PDF

CHAPTER -16 ORGANIC CHEMISTRY The fossil fuels:- The fossil fuels are petroleum (or crude oil), coal and natural gas. They are called fossil fuels because they are the remains of plants and animals that lived millions of years ago. The remains of dead organisms that fell to the ocean floor and were...

CHAPTER -16 ORGANIC CHEMISTRY The fossil fuels:- The fossil fuels are petroleum (or crude oil), coal and natural gas. They are called fossil fuels because they are the remains of plants and animals that lived millions of years ago. The remains of dead organisms that fell to the ocean floor and were buried under thick sediment got slowly converted them to petroleum over millions of years, due to high pressure. Natural gas is mainly methane. It is often found with petroleum. It is formed in the same way. But high temperature and high pressures caused the compounds to break down to gas. Coal is the remains of lush vegetation that grew in ancient swamps. The dead vegetation was buried under thick sediment. Pressure and heat slowly converted it to coal, over millions of years. Petroleum:- Petroleum is a smelly mixture of hundreds of different compounds. They are organic compounds, which means they contain carbon and usually hydrogen. They are mostly hydrocarbons, which means they contain only carbon and hydrogen. Refining:- Petroleum contains hundreds of different hydrocarbons. But a big mixture like this is not very useful. So the first step is to separate the compounds into groups with molecules of a similar size. This is called refining the petroleum. It is carried out by fractional distillation. Refining Petroleum in Laboratory:- The apparatus as below can be used to refine petroleum in the lab. Level-9 Chemistry Organic Chemistry Notes Page 1 of 12  1. As you heat the petroleum, the compounds start to evaporate. The ones with smaller lighter molecules go first, since it takes less energy to free these from the liquid. 2. As the hot vapours rise, so does the thermometer reading. The vapours condense in the cool test-tube. 3. When the thermometer reading reaches 100 °C, replace the first test-tube with an empty one. The liquid in the first test-tube is your first fraction from the distillation. 4. Collect three further fractions in the same way, replacing the test-tube at 150 °C, 200 ° C, and 300 °C. Comparing the fractions:- The larger the molecules in a hydrocarbon – ¸ the higher its boiling point will be ¸ the less volatile it will be ¸ the less easily it will flow (or the more viscous it will be) ¸ the less easily it will burn. Petroleum Refinery:- Level-9 Chemistry Organic Chemistry Notes Page 2 of 12 In a refinery, the fractional distillation is carried out in a tower that is kept very hot at the base, and cooler towards the top. Petroleum is pumped in at the base. The compound start to boil off. Those with the smallest molecules boil off first, and rise to the top of the tower. Others rise only part of the way, depending on their boiling points, and then condense. As the molecules get larger, the fractions get less runny or more viscous. From gas at the top of the tower to solid at the bottom. They also get less flammable so the last two fractions in the table are not used as fuel. Cracking hydrocarbons:- After fractional distillation, the fractions need further treatment before they can be used.  They contain impurities – mainly sulfur compounds. If left in the fuels, these will burn to form harmful sulfur dioxide gas  Some fractions are separated further into single compounds or smaller groups of compounds. For example, the gas fraction is separated into methane, ethane, propane, and butane Part of a fraction may be cracked. Cracking breaks molecules down into smaller ones. Cracking hydrocarbons in laboratory:- Level-9 Chemistry Organic Chemistry Notes Page 3 of 12  ¸ This experiment is carried out using a hydrocarbon oil from petroleum. The product is a gas, collected over water in the inverted test-tube ¸ The moment heating is stopped, the delivery tube must be lifted out of the water. Otherwise water will get sucked up into the hot test-tube Cracking in the refinery:- In the refinery, cracking is carried out in a similar way.  The long-chain hydrocarbon is heated to vaporize it  The vapour is usually passed over a hot catalyst  Thermal decomposition takes place Examples of cracking 1. Cracking the naphtha fraction: Compounds in the naphtha fraction are often cracked, since this fraction is used as the feedstock for making many useful chemicals. This is the kind of reaction that occurs: Level-9 Chemistry Organic Chemistry Notes Page 4 of 12  2. Cracking ethane: Ethane has very short molecules – but even it can be cracked, to give ethene and hydrogen Families of organic compounds:- There are millions of organic compounds. The compounds are named in a very logical way. Level-9 Chemistry Organic Chemistry Notes Page 5 of 12 Comparing families:- Functional groups:- A functional group is the part of a molecule that largely dictates how the molecule will react. For example, all the alkenes have similar reactions because they all have the same functional group, the C = C bond. Homologous series:- In a homologous series,  All the compounds fit the same general formula. For alkanes, the general formula is CnH2n+2, where n is an integer. For methane, n = 1, giving the formula CH4. For ethane, n=2 giving the formula C2H6  The chain length increases by 1 each time.  As the chain gets longer, the compounds show a gradual change in properties. That is,  The melting and boiling points rise  Viscosity increases (compounds flow less easily)  Flammability decreases (compounds burn less easily) Alkanes:- Level-9 Chemistry Organic Chemistry Notes Page 6 of 12 Fig: Bonding in ethane  The alkanes are the simplest family of organic compound.  They are hydrocarbons. They contain only carbon and hydrogen.  Their carbon-carbon bonds are all single bonds.  They form a homologous series, with general formula CnH2n+2 Properties:  They are found in petroleum and natural gas. Petroleum contains alkanes with up to 70 carbon atoms. Natural gas is mainly methane, with small amounts of ethane, propane, butane and other compounds.  The first four alkanes are gases at room temperature. The next twelve are liquids. The rests are solids. Boiling points increase with chain length because attraction between the molecules increases – so it takes more energy to separate them.  Since all their carbon-carbon bonds are single bonds, the alkanes are called saturated.  Generally alkanes are quite unreactive.  Alkanes do burn well in a good supply of oxygen, forming carbon dioxide and water vapour, giving out plenty of heat. So they are used as fuels. Methane burs the most easily. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) + heat energy  If there is not enough oxygen, the alkanes undergo incomplete combustion, giving poisonous carbon monoxide 2CH4 (g) + 3O2 (g) 2CO (g) + 4H2O (l) + less heat energy Level-9 Chemistry Organic Chemistry Notes Page 7 of 12   Alkane also reacts with chlorine in sunlight. This is called substitution reaction, because chlorine atom takes the place of hydrogen atom. If there is enough chlorine, all four hydrogen atoms will be replaced. Isomers:- Isomers are the compounds with same formula, but different structure. Ex: C4H10. Since isomers have different structures, they also have slightly different properties. For example, branched isomer have lower boiling points, because the branches make it harder for the molecules to get close. So the attraction between them is less strong and less heat is needed to overcome it. Alkenes:- Fig: The bonding in Ethene.  The alkenes are hydrocarbons.  They form a homologous series, with general formula, CnH2n  They all contain the C = C double bond. This is their functional group.  Because they contain C =C double bond, they are called unsaturated. Level-9 Chemistry Organic Chemistry Notes Page 8 of 12 Properties:-  The alkenes are made from alkanes by cracking. For example, ethene is formed by cracking ethane. Hydrogen is also produced.  Alkenes are much more reactive than alkanes. Because, the double bond can break to add on other atoms. For example, ethene can add on hydrogen again, to form ethane.  It also adds on water (steam) to form ethanol, an alcohol. The reactions are called addition reactions. An addition reaction turns an unsaturated alkene in to a saturated compound. Polymerization:- Alkene molecules undergo a very useful addition reaction, where they add on to each other to form compounds, with long carbon chains. The alkene molecules are called monomers. The long-chain compounds that form are called polymers. The reaction is called polymerization. Level-9 Chemistry Organic Chemistry Notes Page 9 of 12 The product is polyethene or polythene. Test for unsaturation:- Bromine water to test whether a hydrocarbon is unsaturated. It is an orange solution of bromine in water. If a c=c double bond is present, an addition reaction takes place and colour disappears. Isomers:- In alkenes the chain can branch in different ways and the double bonds can be in different positions. Example, C4H8 Alcohol:-  The alcohols are the family of the organic compound that contain the OH group.  They form a homologous series with the general formula, CnH2n+1OH  Their OH functional group means they will all react in a similar way. Uses:-  Ethanol is the alcohol in alcoholic drinks.  It is a good solvent. It dissolve many substances that do not dissolve in water Level-9 Chemistry Organic Chemistry Notes Page 10 of 12   It evaporates easily – it is volatile. That makes it a suitable solvent to use in glue, printing inks, perfumes, and aftershave. Preparation:- Ethanol is produced in two ways, one biological and one chemical. 1. By fermentation (biological way): Ethanol is made from glucose using yeast, in the absence of air.  Yeast is a mass of living cells. The enzymes in it catalyze the reaction.  The process is called fermentation, and it is exothermic.  Ethanol can be made in this way from any substance that contains sugar, starch, or cellulose. For example, it can be made from sugarcane, maize, potatoes, and wood.  The yeast stops working when the percentage of ethanol reaches a certain level, or if the mixture gets too warm.  The ethanol is separated from the final mixture by fractional distillation. 2. By the hydration of ethene (chemical way):- Hydration means, water is added on. This is an addition reaction.  The reaction is reversible and exothermic.  High pressure and low temperature would give best yield. But in practice the reaction is carried out at 570 degree centigrade to give a decent rate of reaction.  A catalyst is also used to speed up the reaction. Ethanol as a fuel:- Ethanol burns well in oxygen, giving out plenty of heat: It is increasingly used as a fuel for car engines because,  It can be made quite cheaply from waste plant material  Many countries have no petroleum of their own, and have to buy it from other countries; it costs a lot, so ethanol is an attractive option  Ethanol has less impact on carbon dioxide levels than fossil fuels do. Level-9 Chemistry Organic Chemistry Notes Page 11 of 12 Carboxylic acid:- The family forms a homologous series with the general formula CnH2nO2. The functional group COOH is also called the carboxyl group. Preparation:- Ethanoic acid is made by oxidizing ethanol. Oxidation can be carried out in 2 ways: 1. By fermentation – the biological way:- When ethanol is left standing in air, bacteria bring about its oxidation to ethanoic acid. This method is called acid fermentation. Acid fermentation is used to make vinegar (a dilute solution of ethanoic acid). The vinegar starts as food such as apples, rice, honey, which are first fermented to give ethanol. 2. Using oxidizing agents – the chemical way:- Ethanol is oxidized much faster by warming it with the powerful oxidizing agent, potassium manganite(VII), in the presence of acid. The manganite(VII) ions are themselves reduced to Mn2+ ions, with a colour change. The acid provides the H+ ions for the reaction: MnO4- + 8H+ + 5e- Mn2+ + 4H2O Typical acid reactions: 1. A solution of ethanoic acid turns litmus red. 2. A solution of ethanoic acid contains H+ ions, because some of the ethanoic acid molecules dissociate in water. 3. Ethanoic acid reacts with metals, bases, and carbonates, to form salts. Like all Level-9 Chemistry Organic Chemistry Notes Page 12 of 12 salts, sodium ethanoate is an ionic compound. Esters:- Ethanoic acid also reacts with alcohols, to give compounds called esters.  Two molecules have joined to make a larger molecule, with the loss of a small molecule, water. So this is called a condensation reaction.  The reaction is reversible, and sulfuric acid acts a catalyst.  The alcohol part comes first in the name, but second in the formula.  Propyl ethanoate smells of pears. In fact many esters have attractive smells and tastes. So they are added to shampoos and soaps for their smells, and to ice cream and other foods as flavorings. Level-9 Chemistry Organic Chemistry Notes Page 13 of 12