What is the primary difference between a sigma bond and a pi bond in an alkene?

The primary difference lies in their formation and electron density distribution. A sigma ($sigma$) bond is formed by the direct, head-on overlap of atomic orbitals (specifically $sp^2$ hybrid orbitals in alkenes), resulting in electron density concentrated along the internuclear axis. It's a stronger bond. A pi ($pi$) bond, on the other hand, is formed by the sideways overlap of unhybridized $p$ orbitals, with electron density concentrated above and below the internuclear axis. Pi bonds are generally weaker than sigma bonds and are responsible for the restricted rotation and higher reactivity of alkenes.

Why are the carbon atoms in a double bond $sp^2$ hybridized, and what is the resulting geometry?

The carbon atoms in a double bond are $sp^2$ hybridized because they need to form three sigma bonds (one to the other carbon, and two to other atoms like hydrogen or carbon) and one pi bond. To achieve this, one $s$ orbital and two $p$ orbitals mix to form three $sp^2$ hybrid orbitals. These three hybrid orbitals arrange themselves as far apart as possible in a plane, leading to a trigonal planar geometry around each carbon atom, with ideal bond angles of $120^circ$. The remaining unhybridized $p$ orbital is then available for pi bond formation.

What is meant by 'restricted rotation' around a carbon-carbon double bond?

Restricted rotation refers to the inability of the two carbon atoms involved in a double bond to freely rotate relative to each other, unlike in a single bond. This is due to the presence of the pi ($pi$) bond. The pi bond is formed by the sideways overlap of parallel $p$ orbitals. If one carbon atom were to rotate, this overlap would be broken, which requires a significant amount of energy. This energy barrier prevents free rotation at room temperature, leading to the possibility of geometrical isomers (cis-trans isomers) where substituents are fixed in specific spatial arrangements.

How do you number the carbon chain when naming an alkene with multiple double bonds?

When an alkene has multiple double bonds (e.g., a diene or triene), you still select the longest continuous carbon chain that contains *all* the double bonds. The chain is then numbered from the end that gives the double bonds the lowest possible set of numbers. If there's a tie, then substituents are considered to break the tie. The positions of all double bonds are indicated, and the suffix changes to '-diene', '-triene', etc. For example, $CH_2=CH-CH=CH_2$ is 1,3-butadiene.

Can a cyclic alkene have a double bond at any position?

In cyclic alkenes, the double bond is inherently assigned positions 1 and 2. You don't explicitly write 'cyclo-1-hexene' because the double bond's position is understood. Numbering then proceeds around the ring in the direction that gives the lowest possible numbers to any substituents present. For example, in 1-methylcyclohexene, the double bond is between C1 and C2, and the methyl group is on C1. If it were on C3, it would be 3-methylcyclohexene, with the double bond still implicitly between C1 and C2.

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Version 1Updated 22 Mar 2026

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Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond ( $C=C$ ). This double bond consists of one strong sigma ( $sigma$ ) bond and one weaker pi ( $pi$ ) bond. The carbon atoms involved in the double bond are $sp^2$ hybridized, leading to a trigonal planar geometry around each carbon with approximate bond angles of $120^circ$ . The presence of the p…

Quick Summary

Alkenes are unsaturated hydrocarbons characterized by at least one carbon-carbon double bond ( $C=C$ ). Their general formula is $C_nH_{2n}$ . Each carbon in the double bond is $sp^2$ hybridized, resulting in a trigonal planar geometry with bond angles of approximately $120^circ$ .

The double bond consists of one strong sigma ( $sigma$ ) bond, formed by head-on overlap of $sp^2$ orbitals, and one weaker pi ( $pi$ ) bond, formed by sideways overlap of unhybridized $p$ orbitals. The presence of the pi bond restricts rotation around the $C=C$ axis, which is crucial for geometrical isomerism.

IUPAC nomenclature for alkenes involves identifying the longest carbon chain containing the double bond, numbering it to give the double bond the lowest possible number, and replacing the '-ane' suffix with '-ene'.

Substituents are named and located alphabetically. Common examples include ethene and propene. This fundamental understanding of structure and naming is essential for comprehending alkene reactivity and stereochemistry.

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Key Concepts

sp^2

Hybridization and Geometry

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IUPAC Naming of Branched Alkenes

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Sigma and Pi Bond Count in Alkenes

Understanding the number of sigma and pi bonds in an alkene is a common NEET question. Every single bond is a…

Alkenes — Unsaturated hydrocarbons,

C_nH_{2n}

, contain

C=C

Double Bond —

1sigma

bond (head-on

sp^2-sp^2

) +

1pi

bond (sideways

p-p

Hybridization — Carbons in

C=C

are

sp^2

hybridized.

Geometry — Trigonal planar around each

sp^2

carbon, bond angles

approx 120^circ

Rotation — Restricted around

C=C

due to

pi

bond.

Nomenclature (IUPAC)

1. Longest chain *with* $C=C$ . 2. Number for lowest $C=C$ position. 3. Suffix '-ene'. 4. Substituents alphabetical with position.

Bond Lengths —

C=C

(

1.34,\text{Å}

) <

C-C

(

1.54,\text{Å}

To remember the IUPAC naming priority for alkenes: Double Bond Lowest Number. (Double Bond Lowest Number). For the structure of the double bond, think: Sigma Pi Restricted Rotation ( $sp^2$ hybridization, Pi bond, Restricted Rotation).

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