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Hydrocarbons - Alkanes (Preparation and Reactions)

Grade 11ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Alkanes are saturated open-chain hydrocarbons containing carbon-carbon single bonds with the general formula CnH2n+2C_nH_{2n+2}.

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In alkanes, every carbon atom is sp3sp^3 hybridized, forming four sigma bonds in a tetrahedral geometry with a bond angle of 109.5∘109.5^\circ.

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Preparation via Sabatier-Senderens Reaction: Hydrogenation of alkenes or alkynes using catalysts like NiNi, PtPt, or PdPd at specific temperatures.

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Wurtz Reaction: Treatment of alkyl halides with sodium metal in dry ether to produce higher alkanes with an even number of carbon atoms (2RX+2Na→dry etherR−R+2NaX2RX + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX).

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Decarboxylation: Heating sodium salts of carboxylic acids with soda lime (NaOHNaOH and CaOCaO in 3:13:1 ratio) to yield an alkane with one less carbon atom than the parent salt.

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Kolbe's Electrolytic Method: Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid gives alkanes at the anode.

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Substitution Reactions: Alkanes undergo free radical substitution, most notably halogenation in the presence of UVUV light or heat (hνh\nu).

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Combustion: Alkanes burn in oxygen to produce CO2CO_2, H2OH_2O, and a large amount of heat ("ΔH""\Delta H" is negative).

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Isomerization: nn-Alkanes heated with anhydrous AlCl3AlCl_3 and HClHCl gas rearrange to form branched-chain isomers.

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Aromatization: Alkanes with six or more carbon atoms heated to 773K773 K at 10−2010-20 atm over oxides of VV, CrCr, or MoMo get dehydrogenated and cyclized to benzene or its derivatives.

📐Formulae

CnH2n+2C_nH_{2n+2}

R−CH=CH2+H2→Ni/Pt/PdR−CH2−CH3R-CH=CH_2 + H_2 \xrightarrow{Ni/Pt/Pd} R-CH_2-CH_3

2CH3X+2Na→dry etherCH3−CH3+2NaX2CH_3X + 2Na \xrightarrow{\text{dry ether}} CH_3-CH_3 + 2NaX

CH3COONa+NaOH→CaO,ΔCH4+Na2CO3CH_3COONa + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3

CH4+Cl2→hνCH3Cl+HClCH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl

CnH2n+2+(3n+12)O2→nCO2+(n+1)H2O+HeatC_nH_{2n+2} + (\frac{3n+1}{2})O_2 \rightarrow nCO_2 + (n+1)H_2O + \text{Heat}

💡Examples

Problem 1:

Explain why the Wurtz reaction is not preferred for the preparation of unsymmetrical alkanes like propane (C3H8C_3H_8).

Solution:

If a mixture of two different alkyl halides (e.g., CH3BrCH_3Br and C2H5BrC_2H_5Br) is used, three different alkanes are formed: ethane (CH3−CH3CH_3-CH_3), propane (CH3−CH2−CH3CH_3-CH_2-CH_3), and butane (C2H5−C2H5C_2H_5-C_2H_5).

Explanation:

The mixture of these alkanes has very close boiling points, making it extremely difficult to separate the desired unsymmetrical alkane (propane) in pure form, resulting in a low yield.

Problem 2:

Write the chemical equation for the preparation of ethane from sodium propionate using the decarboxylation method.

Solution:

C2H5COONa+NaOH→CaO,ΔC2H6+Na2CO3C_2H_5COONa + NaOH \xrightarrow{CaO, \Delta} C_2H_6 + Na_2CO_3

Explanation:

Sodium propionate reacts with soda lime. The carboxyl group is removed as Na2CO3Na_2CO_3, leaving behind the ethyl group which combines with hydrogen to form ethane (C2H6C_2H_6).

Problem 3:

Identify the major product when nn-hexane is heated with anhydrous AlCl3AlCl_3 and HClHCl.

Solution:

The major products are branched-chain isomers: 22-methylpentane and 33-methylpentane.

Explanation:

This is an isomerization reaction where straight-chain alkanes convert into branched-chain isomers in the presence of AlCl3/HClAlCl_3/HCl catalyst.

Alkanes (Preparation and Reactions) Revision - Class 11 Chemistry ICSE