Revision: Class 11 >> Hydrocarbons NEET (UG) Hydrocarbons

Hydrocarbons are compounds that 9re made up only of carbon and hydrogen atoms.

OR

The compounds which contain carbon and hydrogen as the only two elements are called hydrocarbons.

The valencies of all the atoms are satisfied by single bonds between them. Such carbon compounds are called saturated compounds.

The compounds of carbon having double or triple bonds between the carbon atoms are known as unsaturated carbon compounds.

Alkanes are hydrocarbons in which all the linkages between the carbon atoms are single covalent bonds.

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond.

Alkynes are aliphatic unsaturated hydrocarbons containing at least one C≡C triple bond.

• Hydrocarbons contain only carbon and hydrogen atoms and are the basic type of organic compounds.
• Saturated hydrocarbons have single bonds (e.g. methane, ethane, propane).
• Unsaturated hydrocarbons have double or triple bonds (e.g. ethene, ethyne).
• To draw structures, join the carbon atoms first, then add hydrogen atoms to complete 4 bonds per carbon.
• Ethene has a double bond, and ethyne has a triple bond between carbon atoms.

• General formula: CₙH₂ₙ₊₂ (where n = number of carbon atoms)
• Suffix used for IUPAC naming: –ane
• e.g., Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈)
• Alkanes exhibit chain isomerism due to absence of any functional group and the possibility of more than one chain type for the same molecular formula
• e.g., C₅H₁₂ forms n-pentane, neo-pentane, and iso-pentane

Methods of Preparation:

1. Polarity: 

• Alkanes are non-polar
• Insoluble in polar solvents (water)
• Soluble in non-polar solvents

2. Boiling point: 

Increases with molecular mass

State at SATP:

• C₁–C₄ → gases
• C₅–C₁₇ → liquids
• C₁₈+ → solids

Boiling point order (increasing):

• Neo-pentane < iso-pentane < n-pentane

Trend:

• Straight-chain > branched-chain
• More branching → lower BP

3. Melting point: 

• Increases with molecular mass
• Symmetrical molecules → higher MP
• Even number of C atoms → higher MP than odd
• Only intermolecular London forces are present

Alkanes are saturated and relatively inert — they undergo only substitution reactions at C–H bonds.

Reaction	Conditions
Halogenation	\[\ce{CH4 + Cl2 ->[hv][-HCl] CH3Cl ->[hv][-HCl] CH2Cl2 ->[hv][-HCl] CHCl3 ->[hv][-HCl] CCl4}\]
Combustion	\[\ce{C_{n}H_{2n + 2} + \left(\frac{3n + 1}{2}\right)O_{2} ->[Complete][combustion] nCO2 + (n + 1)H2O}\] \[\ce{CH4(g) + 20_{2}(g) ->[Complete][combustion] CO2 + 2H2O}\]
Reforming / Aromatization	\[\ce{n-hexane ->[V2O5][\underset{12-20 atm}{773K}] C6H6 + 4H2}\]
Pyrolysis / Cracking

Conformations in Alkanes:

Conformation = spatial arrangement of atoms that can be interconverted by rotation about a C–C single bond. Also called conformers or rotamers.

For ethane, two extreme conformations exist:

Conformation	Description	Energy	Stability
Eclipsed	Hydrogen atoms on adjacent carbons are as close as possible	Maximum energy	Highly unstable
Staggered	Hydrogen atoms are as far apart as possible	Minimum energy	Highly stable

 

Skew conformation is an intermediate between eclipsed and staggered conformations. Both are represented by Sawhorse and Newman projections.

Sawhorse formula of ethane

 

Newmann projection formula ethane

• General formula: CₙH₂ₙ (where n = 2, 3, 4…)
• Suffix for IUPAC naming: –ene
• e.g., Ethene (CH₂=CH₂), Propene (CH₃–CH=CH₂)
• The double bond consists of one σ bond and one π bond

1. Solubility

• Alkenes are non-polar
• Insoluble in water
• Soluble in non-polar organic solvents (e.g., hexane, benzene)

2. Boiling Point (BP)

Increases with molecular mass

• More electrons → stronger London dispersion forces

Straight-chain > Branched-chain

• Straight chains have larger surface area → stronger intermolecular forces

Cis-alkenes > Trans-alkenes (usually)

• Cis is more polar → dipoles do not cancel → higher BP

Alkenes vs Alkanes (same number of carbons)

• Alkenes have slightly lower BP

Reason:

• π-bond leads to less effective electron distribution for dispersion forces
• Slightly weaker intermolecular attractions

3. Melting Point (MP)

Trans-alkenes > Cis-alkenes

• Trans is more symmetrical → packs better in crystal lattice → higher MP

Cis-alkenes

• Less symmetrical → poorer packing → lower MP

In trans-alkenes:

• Bond dipoles cancel → non-polar
• Leads to tighter packing in solid state

Alkenes undergo mainly electrophilic addition reactions due to the π bond (electron-rich site).

Reaction	Example
Addition of hydrogen (Hydrogenation)	\[\ce{H2C = CH2 ->[H2/Ni, Pt or Pd][523-573K] H3C - CH3}\]
Addition of halogen	\[\begin{array}{cc} \phantom{}\ce{H3CCH = CH2 + Cl — Cl ->[CCl4] CH3CH - CH2}\phantom{}\\ \phantom{....................................................}|\phantom{.........}|\phantom{}\\ \phantom{.....................................................}\ce{Cl}\phantom{.......}\ce{Cl}\phantom{} \end{array}\]
Addition of HX (Markovnikov's rule)	\[\begin{array}{cc} \phantom{..............................................................}\ce{Br}\phantom{}\\ \phantom{............................................................}|\phantom{}\\ \phantom{}\ce{\underset{(For unsymmetrical allkene-Markownikoff’s rule)}{H3CCH = CH2 + HBr} -> H3C - CH - CH3}\phantom{} \end{array}\]
Addition of HBr (Anti-Markovnikov / Kharasch effect)	\[\begin{array}{cc} \phantom{}\ce{\underset{(In presence of peroxide-reverse of Markownikoff’s rule)}{H3CCH = CH2 + HBr} ->[Peroxide] CH3CH2CH2Br}\phantom{}\\ \end{array}\]
Hydration (addition of H₂SO₄/H₂O)	\[\begin{array}{cc}  \ce{O}\phantom{..}\\ ||\phantom{..}\\  \phantom{}\ce{CH2 = CH2 + H - O - S - O - H -> C2H5HSO4}\phantom{}\\ ||\phantom{..}\\ \ce{O}\phantom{..}  \end{array}\]
Oxidation (KMnO₄/H⁺)	\[\begin{array}{cc} \phantom{..........................}\ce{O}\phantom{}\\ \phantom{..........................}||\\ \phantom{}\ce{H3C — CH = CH2 ->[{[O]}][KMnO4, {[H^{+}]}] H3C - C - OH + CO2 + H2O}\phantom{} \end{array}\]
Hydroxylation	\[\begin{array}{cc} \phantom{}\ce{H2C = CH2 + H2O + [O] ->[Dil.KMnO4][273K] CH2 - CH2}\\ \phantom{.....................................................}|\phantom{..........}|\phantom{}\\ \phantom{........................................................}\ce{OH}\phantom{.....}\ce{OH}\phantom{} \end{array}\]
Ozonolysis	\[\begin{array}{cc} \phantom{.....}\ce{H3C}\phantom{....................................}\ce{H3C}\phantom{............................}\\ \phantom{.....}\backslash\phantom{.........................................}\backslash\phantom{.....................}\\ \phantom{..........}\ce{C = CH2 + O3 ->[Zn/H2O] \phantom{.......}C = O + HCHO}\phantom{}\\ \phantom{......}/\phantom{..........................................}/\phantom{.....................}\\ \phantom{...............}\ce{H3C}\phantom{......................................}\ce{H3C}\phantom{......................................} \end{array}\]
Polymerisation
Hydroboration-oxidation	\[\begin{array}{cc}  \phantom{...............}\ce{H}\phantom{....}\ce{H}\phantom{.............................................}\ce{H}\phantom{....}\ce{H}\phantom{..........................}\ce{H}\phantom{....}\ce{H}\phantom{............................}\\ \phantom{.............}|\phantom{......}|\phantom{..............................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\phantom{..........................}\\ \phantom{}\ce{6(H - C = C - H) + (BH3)2 ->[THF] 2(H - C - C)3 - B ->[H2O2][OH^Θ] H - C - C - H + B(OH)3}\phantom{}\\ \phantom{..........................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\\ \phantom{..............................................}\ce{H}\phantom{.....}\ce{H}\phantom{..........................}\ce{H}\phantom{.....}\ce{OH}\phantom{.} \end{array}\]

• General formula: CₙH₂ₙ₋₂
• Suffix for IUPAC naming: –yne
• e.g., Propyne (CH₃–C≡CH), Butyne (CH₃–CH₂–C≡CH)
• C₂H₂ is acetylene (common name); IUPAC name is ethyne
• The triple bond consists of one σ bond and two π bonds

Isomerism in Alkynes:

• Alkynes show position isomerism (type of structural isomerism)
• e.g., 1-Butyne and 2-Butyne

From calcium carbide:

By the action of water on calcium carbide, Cu₂C₂ or BaC_2, acetylene is formed.

CaC₂ + 2H₂O → CH ≡ CH + Ca(OH)₂

\[\ce{CaCO3 ->[][\underset{}{-CO2}] CaO->[2C + Heat][\underset{}{}] CaC2->[H2O][\underset{}{}] CH ≡ CH}\]

Dehalogenation of 1,1,2,2-tetrahaloalkanes:

On treatment with Zn, tetrahalides get dehalogenated to give alkynes.

\[\ce{R - CBr2 - CHBr2 + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ CH} + 2ZnBr2}\]

\[\ce{R - CBr2 - CBr2 - R + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ C} - R + 2ZnBr2}\]

By dehydrohalogenation of vic and gem dihalides:

Alkynes are prepared by dehydrohalogenation of vic. and gem. dihalides with alc. KOH + NaNH₂ or KNH₂.

\[\ce{R - \underset{β}{C}H2 - CHX2 ->[(i) NaNH2/Δ][(ii) H^{+}] R - C ≡ CH}\]

\[\begin{array}{cc}
\ce{X}\phantom{.......................}\\
\phantom{}|\phantom{.......................}\\
\phantom{}\ce{R - \underset{β}{C}H2 - C - R ->[NaNH2/Δ] R - C ≡ CH}\phantom{}\\
\phantom{}|\phantom{.......................}\\
\ce{X}\phantom{.......................}
\end{array}\]

Odour: Alkynes are generally odourless, but acetylene smells of garlic due to phosphine impurity

Boiling and melting points: Slightly higher than those of corresponding alkenes and alkanes with similar carbon atoms

Reason for higher BP: Linear structure around the triple bond allows electrons to come closer together, resulting in greater London forces

Solubility: Alkynes are soluble in organic solvents like benzene, CCl₄, and ether

Density: Increases with increase in molecular size

Non-polar molecules → insoluble in water

Cyclic, planar hydrocarbons with delocalised π electrons. 

Benzenoids contain benzene ring; Non-benzenoids are aromatic without benzene ring.

Structure of Benzene:

• All 6 C atoms are sp² hybridised
• Unhybridised p-orbitals overlap laterally → delocalised π bonds
• Bond length = 139 pm (uniform, due to resonance)

Huckel's Rule

1. Cyclic and planar
2. Each ring atom has a p-orbital
3. Contains (4n + 2) π electrons (n = 0, 1, 2…)

Examples: Benzene, Naphthalene, Pyridine.

Physical Properties:

• Colourless liquid, sweet smell
• BP = 353 K, MP = 278.5 K
• Immiscible with water; burns with sooty flame

Preparation:

Electrophilic Substitution:

Aromatic compound are cyclic and planar, Aromaticity is due to extensive delocalisation of π-electrons in planar ringsystem.

Condition of Aromatic Nature: A compound will be aromatic if.

1. Compound should be cyclic 
2. Compound should be planer 
3. Compound should be conjugated 

It obey Hukel rule (4n + 2) = ne value of n will be 0, 1,3

• Aromatic hydrocarbons are non-polar molecules and are usually colourless liquids or solids with a characteristic aroma. 
• Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.

(i) Electrophilic substitution:

(ii) Addition reaction:

(iii) Friedel-Crafts reactions:

(iv) Friedel-Crafts acylation:

Aromatic hydrocarbons are electron-rich → give electrophilic substitution reactions (act as nucleophiles).

• Aromaticity is preserved in ESR because the double bond is not broken.
• E is the electrophile.

Electrophilic substitution reaction takes place in three steps: 

• Generation of electrophile
• Formation of carbocation
• Removal of proton

Step 1: Generation of Electrophile

In the first step of the electrophilic substitution reaction, the electrophile is generated with the help of a Lewis acid example H⁺, FeX₃, or AICl₃

Step 2: Electrophile attack on benzene ring

The positive bromine ion attacks benzene to form an arenium ion.

Step 3: Deprotonation from benzene ring

A proton is removed from the arenium ion to become bromobenzene.

• Benzene and polynuclear hydrocarbons (containing two or more fused benzene rings) are toxic and may show carcinogenic (cancer-causing) properties.
• Some polynuclear hydrocarbons like benzo[a]pyrene are strongly carcinogenic.
• Anthracene itself is NOT carcinogenic, but some of its derivatives may be.
• Therefore, compounds containing benzene rings should be handled with care.