Free Chemistry NEET Notes for Hydrocarbons

Free Chemistry Notes available for Hydrocarbons (NEET)

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- Hydrocarbons -

Important Chemistry Notes for IITJEE/NEET Preparation- Hydrocarbons

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Hydrocarbons

SATURATED HYDROCARBONS, PARAFFINS OR ALKANES

They are open chain compounds of carbon and hydrogen having all the atoms linked together by single covalent bonds, least reactive in nature

(Parum = little; affinis = affinity) and having general formula CnH2n+2

NOMENCLATURE

TRIVIAL NAMES

The first four members have the trivial names derived from their preparation from corresponding alcohols containing same number of carbon atoms eg. methane from methyl alcohol, ethane from ethyl alcohol and so on. After butane they are named according to Latin or Greek numerals of the number of Carbon atoms present in them with class suffix-ane. eg. Pentane (penta = 5) : hexane (hexa = 6) and so on.

The straight chain hydrocarbons are called normal (n) and contain 1° or 1° & 2° carbon atoms. The branched chain hydrocarbons containing the group are called Iso and contain 1° & 3° or 1°, 2° and 3° carbon atoms. The hydrocarbons containing a quaternary carbon atom are called neo (new).

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neet chemistry notes for hydrocarbons class 11

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DERIVED NAMES

In this system the higher hydrocarbons are considered as derived from methane, eg.

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IUPAC SYSTEM

Types of carbon atoms :

Primary (1°) - It is attached to one carbon atom only.

Secondary (2°) - It is attached to two carbon atoms.

Tertiary (3°) - It is attached to three carbon atoms.

Quaternary (4°) - It is attached to four carbon atoms.

ISOMERISM

They exhibit chain isomerism

Number of isomers of alkanes :

OCCURRENCE

The crude petroleum and natural gas contain hydrocarbons from C1 to C40. Ozokerite (a neutral wax) is a mixture of higher solid hydrocarbons. Waxes of some plants and animals also contain some higher paraffins

GENERAL METHODS OF PREPARATION OF ALKANES

From unsaturated hydrocarbons : (Sabatier and Senderens reaction)

By heating anhydrous sodium salt of fatty acid with soda lime (Lab method).

By reduction of alkyl halides with nascent hydrogen.

Reducing agents used are Na/C2H5OH; Zn/HCl;

Zn–Cu couple; Hg–Al couple, HI/P.

Wurtz synthesis :

Alkanes with odd number of carbon atoms can not be prepared by this method.

Different alkyl halides give mixture of hydrocarbons viz. R – R, R' – R' & R – R'.

3° alkyl halides give alkenes by dehydrohalogenation.

By Frankland's method :

From Grignard's reagent :

H must be attached to O, N, S or sp hybridised C-atom. (i.e. active H-atom).

Kolbe's electrolytic method :

By Clemmensen reduction : From carbonyl compounds

By Wolf. Kishner reduction : From Carbonyl compounds

By Berthelot's reaction :

From metal carbides :

From carbon monoxide

By reduction of alcohols, aldehydes, ketones and carboxylic acids :

Corey-House alkane synthesis :

The reaction is particularly useful for preparing unsymmetrical alkanes.

PROPERTIES

C1–C4 gases; C5–C17 colourless liquids, higher solids. Insoluble in water soluble in organic solvents

Branched chain alkanes boil at lower temperature than isomeric straight chain alkanes. The latter have higher van der Waals forces of attraction.

Melting points of hydrocarbons containing even number of C-atoms are relatively more than those containing odd number of carbon atoms.

CONFORMATIONAL ANALYSIS

Different arrangements of atoms in a molecule convertible into one another by rotation about single bond are called conformers or conformations. Their study is known as conformational analysis. If the energy barrier to the rotation is nil or small, the rotation is said to be free or almost free. Ethane can exist in an infinite number of conformations.

Ө = It is dihedral angle between C–H bonds on the front of a Newman projection and those on the back.

Order of stability : Staggered > Skew > Eclipsed

Energy barrier between eclipsed and staggered is 2.8 kcal/mol.

Conformations of Propane C3H8 :

Order of stability : Staggered > Skew > Eclipsed

Energy barrier between eclipsed and staggered is 3.3 kcal/mol

Conformations of n-Butane C4H10 :

Order of stability : Anti > Gauche > Skew > Eclipsed > Fully Eclipsed

Energy barrier between fully eclipsed and fully staggered (anti) is 5.3 kcal / mol or 22 kJ/mol.

Conformations of Cyclohexane :

It exists in two nonplanar, strainless forms, the boat and the chair form.

Equatorial hydrogens lie in the plane of the ring carbons.

Axial hydrogens lie (up or down) the plane of the ring.

There are six equatorial and six axial hydrogens. In the flipping and reflipping between conformations, axial becomes equatorial and vice versa. Chair form has the lowest energy.

Cyclohexane can assume other shapes also.

BAEYER'S STRAIN THEORY

When ring compounds are formed, the bonds deviate from normal positions which produces a condition of strain in the molecule. The strain is directly proportional to angle of deviation and can be calculated as follows. The more the angle of deviation, the less is the stability.

Angle of deviation of cyclic compound =

For Cyclopropane =

Cyclobutane =

Cyclopentane =

Cyclohexane =

Cycloheptane =

Limitations : Baeyer's strain theory fails to explain the stability of large ring alicyclic compounds.

SACHSE MOHR THEORY OF STRAINLESS RINGS

A ring with six or more carbon atoms can assume "puckered" structure and there is a little distortion of normal tetrahedral angle. Thus there is little or negligible angle strain in the molecule.

UNSATURATED HYDROCARBONS, OLEFINS OR ALKENES

They are open chain compounds of carbon and hydrogen having double bonds also known as Olefins (oil forming) Olefiant gas is Dutch name of ethylene which formed oily ethylene chloride with chlorine. Their general formula is CnH2n.

NOMENCLATURE

COMMON SYSTEM : The suffix -ane of alkane is replaced by -ylene, and named as Alkylenes eg. ethylene, propylene etc.

IUPAC SYSTEM : The suffix -ane of alkane is replaced by -ene and hence named as Alkenes.

DERIVED NAMES : They are named as substituted derivatives of ethylene.

ISOMERISM

Alkenes show four types of isomerism

Chain Isomerism

Position Isomerism

Ring Chain Isomerism

Geometrical Isomerism

GENERAL METHODS OF PREPARATIONS OF ALKENES

By dehydration of alcohols :

Dehydrating agents Conc. H2SO4; P2O5; H3PO4; Al2O3. Anhy. ZnCl2. Anhy. Oxalic acid.

Ease of dehydration of alcohols 3° > 2° > 1°.

By Dehydrohalogenation of Alkyl halides.

Dehydrohalogenation and dehydration follows Saytzeff's rule (Hydrogen is removed from C-atom containing lesser number of H-atoms or more substituted alkene is formed)
More substituted alkenes are more stable.
Ease of dehydrohalogenation 3° > 2° > 1°.
Ease of dehydrohalogenation : Iodides > Bromides > Chlorides

By dehalogenation of vicinal halides :

By dehalogenation of gem. halides :

By electrolysis of Sodium or potassium salt of succinic acid or its derivatives

Alkenes with odd or even number of carbon atoms and having any position of double bond can be prepared.

By partial reduction of alkynes :

Decomposition of quaternary ammonium hydroxide:

By cracking of alkanes :

PROPERTIES

C2–C4 gases; C4 – C15 liquids C16 onwards Solids. Less volatile than alkanes and possess anaesthetic properties.

CHEMICAL PROPERTIES

Addition reactions given by alkenes are known as electrophilic addition reactions.

ALKYNES OR ACETYLENES

They are characterised by the presence of triple bond having general formula CnH2n–2.

NOMENCLATURE

According to trivial system they are regarded as derivatives of acetylene. In the IUPAC system their names are derived by replacing suffix -ane by -yne.

Formula	Common Name	Derived name	IUPAC name
CH ☰ CH	Acetylene	Acetylene	Ethyne
CH3C ☰ CH	Allylene	Methyl acetylene	Propyne
CH3C ☰ C– CH3	Cretonylene	Dimethyl acetylene	But-2-yne

ISOMERISM

They exhibit four types of Isomerism

Chain Isomerism

Hex–1–yne 4–methyl pent –1– yne

Position Isomerism

Hex – 1–yne Hex – 3 – Yne

Functional Isomerism

But – 1 –yne But –1,3 – diene

Ring chain Isomerism

But – 2 –yne cyclobutene

GENERAL METHODS OF PREPARATION

By dehydrohalogenation of vicinal halides

By dehydrohalogenation of gem halides

By dehalogenation of tetrahalides

From lower alkynes

NaC

CNa

R – C

C – R + 2NaX

Kolbe's electrolytic method

From haloform

CHI3 + 6Ag + I3CH

CH + 6AgI

Acetylene from calcium carbide (wohler's reaction)

CaC2 +2H2O

Ca(OH)2 + CH

Berthlot's reaction

From methane :

PROPERTIES

C2–C4 gases C5–C12 liquids. C13 onwards solids. Acetylene has garlic odour due to phosphene and hydrogen sulphide impurity.

CHEMICAL PROPERTIES

Alkynes are less reactive than alkenes for electrophilic addition reactions.

AROMATIC HYDROCARBONS, BENZENE C6H6

PREPARATION

SMALL SCALE PREPARATION

From acetylene :

From benzoic acid :

From Phenol :

From Chlorobenzene :

From benzene sulphonic acid :

From benzene diazonium chloride :

LARGE SCALE PREPARATION

From Petroleum : n-hexane fraction of petroleum

+ 4H2

From light-oil fraction of Coal tar

Impurity of thiophene is removed by heating with hydrogen under pressure at 400°C in presence of catalyst.

PROPERTIES

Colourless Liquid : bpt 80.1°C. Insoluble in H2O.

STRUCTURE OF BENZENE :

Open chain structures proposed.

Ring structure by Kekule

Objections against Kekule structure :

Two ortho di substituted derivatives

About Stability

Explanation against above objections :

Presence of two structures undergoing quick interconversion.

Levine and Cole : Levine and Cole confirmed the presence of above structures by ozonolysis of o-Xylene and obtaining the products.

Dimethyl glyoxal
Methyl glyoxal and
Glyoxal

Ladenburg's prism structure :

X-ray crystallographic methods showed benzene a planar compound whereas above structure is non planar.

Claus and Dewar's structures :

Baeyer and Armstrong's centric structure :

Robinson's sixtet structure

Thiele's structure of partial valency
:
Benzene and resonance : Benzene is a resonance hybrid of the following structures.

All C–C bonds in the benzene are of the same length 1.39Å.

Benzene and aromaticity : Benzene is an aromatic compound since it obeys Huckel rule of (4n + 2)p electrons. The value of n must be an integer.

4n + 2 = 6p electrons of benzene

Molecular orbital structure of benzene : Each C-atom in benzene is sp2 hybridised and forms 3s bonds. The pz atomic orbitals left on each C-atom form a delocalised p-molecular orbital which stabilises the structure and renders all C-C bonds equal in length.

PETROLEUM

(petra = rock, oleum = oil). A thick dark coloured complex liquid, mixture of organic compounds obtained from below the surface of the earth is petroleum. The chief components of petroleum are hydrocarbons, aliphatic, alicyclic (naphthalenes) or aromatic in varying proportions and 1 to 6 percent of Sulphur, Nitrogen and Oxygen compounds.

NATURAL GAS

Found along with petroleum and roughly contains 60 to 80 percent methane 5 to 9 percent ethane, 3 to 18 percent propane, 2 to 14 percent higher hydrocarbons. It is used as fuel.

Partial combustion of natural gas yields Carbon blocks (reinforcing agents for rubber).

THEORIES OF ORIGIN

MENDELEEV'S CARBIDE THEORY

The theory was supported by Moissan, Sabatier and Senderens. It fails to explain the presence of optically active compounds, Compounds of N and S, chlorophyll and haemin derivatives.

ENGLER'S THEORY

Petroleum is a product of slow decomposition of dead marine animals under high temperature and pressure. It explains the presence of brine, fossils, compounds of N and S.

It explains the presence of all the above mentioned compounds including chlorophyll. Hence petroleum is of animal as well as vegetable origin.

MODERN THEORY

Petroleum is produced by partial decomposition of marine animals and sea weeds etc.

Mining and Refining : Recovery from oil wells and separation of individual components.

FLASH POINT

The minimum temperature at which an oil gives off sufficient vapours to form an explosive mixture with air is called Flash Point.

KNOCKING

A sharp metallic sound emitted by internal combustion engine owing to immature ignition of the air gasoline mixture. Knocking is maximum in presence of straight chain hydrocarbons and minimum in presence of aromatic and branched chain hydrocarbons.

Anti Knock Compounds : 59% Tetraethyl lead, 13% Ethylene bromide, 24% Ethylene chloride, 4% Kerosene and dye is anti knock mixture.

In presence of aromatic compounds tetramethyl lead is more effective.

OCTANE NUMBER

The percentage of isooctane (2, 2, 4-trimethyl pentane) in a mixture of iso octane and n-heptane having the same knocking properties as the fuel under consideration.

The higher the octane number, the better is the fuel.

Octane number of compounds : Aromatic Compounds > Cyclo alkanes > Olefins > Branched chain alkanes > Straight Chain alkanes.

CETANE NUMBER

It is the percentage of cetane (n-hexadecane) in a cetane and a-methylnaphthalene mixture that has the same ignition qualities as the fuel.

CRACKING

The conversion of less valuable higher fraction to the more valuable lower fraction by the application of heat is known as cracking.

Liquid Phase Cracking :

Catalyst : Silica, titanium dioxide, zinc oxide, ferric oxide, alumina etc. High pressure keeps the oil in liquid state. Octane number of product : 65-70.

Vapour phase Cracking :

Cracking in presence of hydrogen :

Reactions taking place during cracking :

Carbonisation :
Dehydrogenation :
Polymerisation :

Alkylation :

Aromatisation :

+ 4H2

Chain Fission :

SYNTHETIC PETROL

BERGINS PROCESS

Heavy oil is reused.

FISCHER-TROPSCH PROCESS

Catalyst - Cobalt (100 parts), thoria (5 parts), magnesia (8 parts), Kieselguhr (200 parts).

REFORMING

It is increasing of antiknock properties by special type of cracking which includes Alkylation, Isomerisation, Aromatisation, Cyclisation, Dehydrogenation, Fractionation etc.

No lead petrol : It does not contain lead and obtained by reforming.

Petro chemicals : Chemicals derived from petroleum sources.