HYDROCARBONS PART 3 || SRO8981 ||SRO

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In this blog page we will discuss about Alkanes and Nomenclature and isomerism

ALKANES

→As already mentioned alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds.

→Methane (CH4) is the first member of this family. Methane is a gas found in coal mines and marshy places.

→ If you replace one hydrogen atom of methane by carbon and join the required number of hydrogens to satisfy the tetravalence of the other carbon atom, what do you get? You get C2H6.

→This hydrocarbon with molecular formula C2H6 is known as ethane.
Thus you can consider C2H6 as derived from CH4 by replacing one hydrogen atom by -CH3 group.

→ Go on constructing alkanes by doing this theoretical exercise i.e., replacing hydrogen atom by –CH3 group.

→ The next molecules will be C3H8, C4H10 …

→These hydrocarbons are inert under normal conditions as they do not react with acids, bases and other reagents.

→ Hence, they were earlier known as paraffins (latin : parum, little; affinis, affinity).
→ Can you think of the general formula for alkane family or homologous series?
→ If we examine the formula of different alkanes we find that the general formula for alkanes is CnH2n+2.

→It represents any particular homologue when n is given appropriate value. Can you recall the structure of methane? According to VSEPR methane has a tetrahedral structure (Fig. 13.1), in which carbon atom lies at the centre and the four hydrogen atoms lie at the four corners of a regular tetrahedron.

→All H-C-H bond angles are of 109.5°.

→In alkanes, tetrahedra are joined together in which C-C and C-H bond lengths are 154 pm and 112 pm respectively . You have already read that C–C and C–H σ bonds are formed by head-on overlapping of sp 3 hybrid orbitals of carbon and 1s orbitals of hydrogen atoms.

NOMENCLATURE AND ISOMERISM

→Nomenclature and isomerism in alkanes can further be understood with the help of a few more examples.

→Common names are given in parenthesis. First three alkanes – methane, ethane and propane have only one structure but higher alkanes can have more than one structure.

→Let us write structures for C4H10. Four carbon atoms of C4H10 can be joined either in a continuous chain or with a branched chain in the following two ways : Fig. 13.1 Structure of methane

→In how many ways, you can join five carbon atoms and twelve hydrogen atoms of C5H12? They can be arranged in three ways as shown in structures III–V

→Structures I and II possess same molecular formula but differ in their boiling points and other properties.

→Similarly structures III, IV and V possess the same molecular formula but have different properties.

→Structures I and II are isomers of butane, whereas structures III, IV and V are isomers of pentane.

→Since difference in properties is due to difference in their structures, they are known as structural isomers. →It is also clear that structures I and III have continuous chain of carbon atoms but structures II, IV and V have a branched chain.

→Such structural isomers which differ in chain of carbon atoms are known aschain isomers.

→Thus, you have seen that C4H10 and C5H12 have two and three chain isomers respectively.

→Based upon the number of carbon atoms attached to a carbon atom, the carbon atom is termed as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°).

→Carbon atom attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called primary carbon atom.

→Terminal carbon atoms are always primary. Based upon the number of carbon atoms.

→ Carbon atom attached to two carbon atoms is known as secondary.

→Tertiary carbon is attached to three carbon atoms and neo or quaternary carbon is attached to four carbon atoms. →Can you identify 1°, 2°, 3° and 4° carbon atoms in structures I to V ?

→ If you go on constructing structures for higher alkanes, you will be getting still larger number of isomers.

→ C6H14 has got five isomers and C7H16 has nine.

→As many as 75 isomers are possible for C10H22. →In structures II, IV and V, you observed that –CH3 group is attached to carbon atom numbered as 2.

Thats all over

● Guys In Next Page Or Blog We Will Understand The Properties Of Alkanes.

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