Chemistry · Organic Chemistry

Hydrocarbons formulas for JEE

Every Hydrocarbons formula you need for JEE, grouped by concept.

FormulasRevision notes
13 formulas5 concepts
01Alkanes02Alkenes03Alkynes04Aromatic Hydrocarbons05Hydrocarbon Reactions
01

Alkanes

4 formulas

General Formula of Alkanes

CnH2n+2C_n H_{2n+2}

The general molecular formula for non-cyclic saturated hydrocarbons (alkanes).

applies whenn is an integer >= 1.
alkanesgeneral_formula

General Formula of Alkyl Groups

CnH2n+1C_n H_{2n+1}

The general formula for an alkyl substituent group derived from an alkane by removal of one hydrogen.

applies whenn is an integer >= 1.
alkylsubstituent

Number of Structural Isomers of Alkanes

N=2n−4+1N = 2^{n-4} + 1

Empirical formula to calculate the number of structural isomers of non-cyclic alkanes.

applies whenValid only for alkanes with n=1 to 7 carbon atoms.
isomersalkanesjee-advanced

Torsional Strain Barrier in Ethane

ΔE≈12.5 kJ mol−1\Delta E \approx 12.5 \text{ kJ mol}^{-1}

The energy difference between the eclipsed (maximum energy) and staggered (minimum energy) conformations of ethane.

applies whenStandard room temperature conditions.
conformationethaneenergy_barrier
02

Alkenes

2 formulas

General Formula of Alkenes

CnH2nC_n H_{2n}

The general molecular formula for acyclic alkenes containing one double bond.

applies whenn is an integer >= 2.
alkenesgeneral_formula

Degree of Unsaturation (DBE)

DBE=C+1−H2+N2−X2DBE = C + 1 - \frac{H}{2} + \frac{N}{2} - \frac{X}{2}

Calculates the number of rings and pi bonds in a molecule (Double Bond Equivalent).

applies whenC=carbons, H=hydrogens, N=nitrogens, X=halogens. Oxygen and sulfur are ignored.
unsaturationdbejee-advanced
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03

Alkynes

1 formula

General Formula of Alkynes

CnH2n−2C_n H_{2n-2}

The general molecular formula for acyclic alkynes containing one triple bond.

applies whenn is an integer >= 2.
alkynesgeneral_formula
04

Aromatic Hydrocarbons

1 formula

Hückel's Rule for Aromaticity

Nπ=4n+2N_{\pi} = 4n + 2

Formula for the number of delocalised pi electrons required for a planar, cyclic, conjugated system to be aromatic.

applies whenn must be a non-negative integer (0, 1, 2, ...).
aromaticityhuckelpi_electrons
05

Hydrocarbon Reactions

5 formulas

Molar Mass of Saturated Aldehyde/Ketone

M=14n+16 uM = 14n + 16 \text{ u}

Used to determine the carbon count of an ozonolysis cleavage product based on its molar mass.

applies whenn is the number of carbon atoms in the resulting aliphatic aldehyde or ketone.
ozonolysismolar_massexercises

Combustion Equation of Alkanes

CnH2n+2+(3n+12)O2→nCO2+(n+1)H2OC_n H_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O

General balanced chemical equation for the complete combustion of alkanes.

applies whenComplete combustion in excess oxygen.
combustionalkanesstoichiometry

Enthalpy of Combustion of Butane

ΔcH⊖=−2875.84 kJ mol−1\Delta_c H^\ominus = -2875.84 \text{ kJ mol}^{-1}

Standard heat of combustion for butane.

applies whenStandard conditions (298K, 1 bar).
combustionbutaneenthalpy

Enthalpy of Combustion of Methane

ΔcH⊖=−890 kJ mol−1\Delta_c H^\ominus = -890 \text{ kJ mol}^{-1}

Standard heat of combustion for methane.

applies whenStandard conditions (298K, 1 bar).
combustionmethaneenthalpy

Combustion Equation of Hydrocarbons

CxHy+(x+y4)O2→xCO2+y2H2OC_x H_y + \left(x + \frac{y}{4}\right)O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O

General balanced chemical equation for the complete combustion of any arbitrary hydrocarbon.

applies whenComplete combustion in excess oxygen.
combustionhydrocarbonsstoichiometry
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