MODULE-16 YDROARBONS SATURATED AND UNSATURATED YDROARBONS:- ydrocarbons can be classified according to the types of bonds between the carbon atoms: (i) Saturated hydrocarbons have only single bonds between carbon atoms. (ii) Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms. Alkanes are saturated hydrocarbons. Alkenes (which have at least one double bond) and alkynes (which have at least one triple bond) are unsaturated. Some important characteristics of saturated hydrocarbons:- 1. These organic compounds contain single carbon-carbon covalent bond. 2. Due to the presence of all single covalent bonds, these compounds are less reactive. 3. Saturated compounds undergo substitution reactions. 4. The number of hydrogen atoms is more when compared to its corresponding unsaturated hydrocarbon. Some important characteristics of unsaturated hydrocarbons:- 1. These organic compounds contain at least one double or triple covalent bond between carbon atoms. 2. Due to the presence of double and triple bonds, these compounds are more reactive. 3. Unsaturated compounds undergo addition reactions. 4. The number of hydrogen atoms is less when compared to its corresponding unsaturated hydrocarbon. OMOLOGOUS SERIES:- A homologous series is a family of organic compounds that: (i) have the same general formula, (ii) have similar chemical properties, and, (iii) show a gradual increase in physical properties such as melting point and boiling point.
ALKANES: - The alkanes are saturated hydrocarbons. Molecules of these compounds contain only single bonds between the carbon atoms in the chain. Alkanes have the general formula; n 2n+2.The simplest hydrocarbon and the simplest alkane is methane. ALKENES: - The alkenes are unsaturated hydrocarbons. Molecules of these compounds contain atleast one = double bond somewhere in the chain. Alkenes have the general formula; n 2n. The simplest alkene has two carbon atoms, and is called ethene.
ALKYNES: - The alkenes are unsaturated hydrocarbons. Molecules of these compounds contain atleast one triple bond somewhere in the chain. Alkynes have the general formula; n 2n-2. The simplest alkyne has two carbon atoms, and is called ethyne. ALKYL GROUP: A functional group formed by removing a hydrogen atom from an alkane molecule. Alkyl groups are abbreviated as "R" in structural formulas. Alkane Formula Alkyl group Formula methane 4 methyl group - 3 ethane 3 3 ethyl group - 2 3 propane 3 2 3 n-propyl group - 2 2 3 butane 3 2 2 3 n-butyl group - 2 2 2 3 Distinguishing alkenes and alkanes:- Alkanes burn in oxygen with a clean flame while alkenes usually produce a sooty flame. This is, however, only a rough test. The main test to distinguish alkenes from alkanes uses bromine water, a reddish-brown solution containing bromine (Br 2 ). Alkanes do not react with bromine water at room temperature but alkenes do. The bromine water becomes colourless when alkenes are added. By contrast, bromine water does not change colour when alkanes are added.
Bromine water may not become fully decolourised. It may lighten only slightly if there are not enough alkenes to react with all the bromine. Functional Group: An atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compound.
In a hydrocarbon chain, one or more hydrogens can be replaced by elements such as halogens, oxygen, nitrogen and sulphur maintaining the valency of carbon equal to four. In such compounds, the element replacing hydrogen is referred to as a heteroatom. Naming a carbon compound:- (i) Identify the number of carbon atoms in the compound. A compound having three carbon atoms would have the name propane. (ii) In case a functional group is present, it is indicated in the name of the compound with either a prefix or a suffix (as given in table shown above). (iii) If the name of the functional group is to be given as a suffix, the name of the carbon chain is modified by deleting the final e and adding the appropriate suffix. For example, a three-carbon chain with a ketone group would be named in the following manner Propane e = propan + one = propanone. (iv) If the carbon chain is unsaturated, then the final ane in the name of the carbon chain is substituted by ene or yne as given in table shown above. For example, a three-carbon chain with a double bond would be called propene and if it has a triple bond, it would be called propyne. Example- (I) Five structural isomers of hexane (i) n-hexane
(ii) Let us try to name the below given isomer of pentane: a) Longest chain is identified. ere it is five carbon chain- pentane. b) Substituent if any is identified. ere it is methyl group. c) Now the chain is numbered in such a way that the carbon bearing the substituent is given the lowest number. ere the carbon bearing methyl group is given the number 2. d) NAME: 2-Methyl pentane. (iii) 3-Methyl pentane (iv) 2, 3-Dimethyl butane (v) 2, 2-Dimethyl butane
(II) Isomers of n-pentane: n-pentane 2-Methyl butane 2,2-Dimethyl propane Most of the fuels we use are either carbon or its compounds: Most carbon compounds release a large amount of heat and light on burning (combustion). 4 + 2 O 2 O 2 + 2 2 O + heat and light 3 2 O + 3 O 2 2 O 2 + 3 2 O + heat and light Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke (soot). owever, limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame. The gas/kerosene stove used at home has inlets for air so that a sufficiently oxygenrich mixture is burnt to give a clean blue flame. If the bottoms of cooking vessels get blackened, it means that the air holes are blocked and fuel is getting wasted. Fuels such as coal and petroleum have some amount of nitrogen and sulphur in them. Their combustion results in the formation of oxides of sulphur and nitrogen which are major pollutants in the environment. Lewis dot structures of some organic compounds: Methane Ethane Ethene
Ethanol Propene arbon tetrachloride Ammonia Ethyne Methanol Nomenclature of some carbon compounds: 1- hloro propane 1- Bromo propane Propan-1-ol Propan- 1-al Propan-2-one Propan-1-oic acid Prop-1-yne
Structures of cyclohexane and benzene: yclohexane ( 6 12 ) Benzene ( 6 6 )
Some textual questions answered: MODULE-16 1) Which of the following hydrocarbons undergo addition reactions? 2 6, 3 8, 3 6, 2 2, 4 Unsaturated hydrocarbons undergo addition reactions. Being unsaturated hydrocarbons, 3 6 and 2 2 undergo addition reactions. 2) Draw the structures for the following compounds. (i) Ethanoic acid (ii) Bromopentane (iii) Butanone (iv) exanal Ethanoic acid: Bromopentane (1-Bromo pentane): Butanone (Butan-2-one): exanal (exan-1-al):
3) Draw the structural isomers of bromo pentane ( 5 11 Br). 1- bromo pentane 2- bromo pentane 3- bromo pentane 4) ow would you name the following compounds? (i) Methanal (Formaldehyde) (ii) exyne (ex-1- yne) (iii) 1- bromo ethane 3 2 Br 5) Why is the conversion of ethanol to ethanoic acid an oxidation reaction? Since the conversion of ethanol to ethanoic acid involves the addition of oxygen to ethanol, it is an oxidation reaction. 6) A mixture of oxygen and ethyne is burnt for welding. an you tell why a mixture of ethyne and air is not used? When ethyne is burnt in air, it gives a sooty flame. This is due to incomplete combustion caused by limited supply of air. owever, if ethyne is burnt in sufficient supply of oxygen, it gives a clean flame with temperature 3000 because of complete combustion. This oxyacetylene flame is used for welding. It is possible to attain such a high temperature only when the combustion is complete. This is the reason why a mixture of ethyne and air is not used.
7) Give a test that can be used to differentiate chemically between butter and cooking oil. Butter is saturated carbon compound while cooking oil is unsaturated carbon compound. An unsaturated carbon compound decolorizes bromine water while a saturated hydrocarbon does not decolorize bromine water. Thus, bromine water (reddish brown in colour) can be used to differentiate between butter and cooking oil. If the given sample does not decolorize the bromine water, it is butter and the one which decolorizes bromine water, is cooking oil.
MODULE-16 (WORKSEET-1) YDROARBONS 1) What is organic chemistry?. 2) Organic compounds that contain only carbon and hydrogen are called. 3) Is the following sentence true or false? Alkanes contain only single covalent bonds.. 4) What is the simplest alkane?. 5) The names of all alkanes end with the suffix. 6) Match the name of the straight-chain alkane with the number of carbon atoms it contains. ydrocarbon Number of carbon atoms exane 7 Propane 4 eptane 3 Butane 6 7) Which family of hydrocarbons are always saturated compounds?. 8) ydrocarbons that contain one or more covalent bonds between carbons are called alkynes. 9) is the simplest alkyne, and is also known by the common name. 10) What is an alkene?. 11) omplete the table below: Number of carbon Alkane [ n 2n+2 ] Alkene [ n 2n ] Alkyne [ n 2n-2 ] atoms (n) 2 2 2x2+2 = 2 6 2 2x2 = 2 4 2 2x2-2 = 2 2 3 4 5 6
MODULE-16 (WORKSEET-2) YDROARBONS-ISOMERS 1) What are structural isomers?. 2) Is the following sentence true or false? Structural isomers have the same physical properties.. 3) Butane has an isomer called methylpropane. Pentane has an isomer called methylbutane. Draw the structural formulae for the given isomers. 4) Identify the functional groups on the following organic molecules. O O 5) Draw the following organic compounds, and state the organic family they belong to. i) Propanol ii) Propyne iii) But-2-ene
iv) Pentan-1-oic acid v) eptan-1 al vi) exan-3-one 6) Name the following organic compounds. ircle the functional group if one is present. i) O ii) O O iii) O iv) O v) vi)
MODULE-16 (WORKSEET-3) YDROARBONS 1) Ethane, with the molecular formula 2 6 has (a) 6 covalent bonds. (b) 7 covalent bonds. (c) 8 covalent bonds. (d) 9 covalent bonds. 2) Butanone is a four-carbon compound with the functional group (a) carboxylic acid. (b) aldehyde. (c) ketone. (d) alcohol. 3) While cooking, if the bottom of the vessel is getting blackened on the outside, it means that (a) the food is not cooked completely. (b) the fuel is not burning completely. (c) the fuel is wet. (d) the fuel is burning completely. 4) The compounds that are formed by the reaction of an alcohol with a carboxylic acid in the presence of concentrated sulphuric acid are known as (a) ethers. (b) esters. (c) aldehydes. (d) ketones. 4) The general formula of alkene series is (a) n 2n (b) n 2n+2 (c) n 2n 2 (d) n 2n+1 5) The IUPA name of the following organic compound is (a) 4- bromo propane (c) 2- bromo propane (b) 1- bromo propane (d) 3- bromo propane