Nomenclature, Structure of Double Bond — Revision Notes

⚡ 30-Second Revision

Alkenes — Unsaturated hydrocarbons, $C_nH_{2n}$, contain $C=C$.
Double Bond — $1\sigma$ bond (head-on $sp^2-sp^2$) + $1\pi$ 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 — $C=C$ ($1.34\,\text{Å}$) < $C-C$ ($1.54\,\text{Å}$).

2-Minute Revision

Alkenes are hydrocarbons featuring at least one carbon-carbon double bond, giving them the general formula $C_nH_{2n}$. The unique structure of this double bond is key: each carbon involved is $sp^2$ hybridized.

This hybridization results from the mixing of one $s$ and two $p$ orbitals, forming three $sp^2$ hybrid orbitals arranged in a trigonal planar geometry with bond angles of about $120^\circ$. The remaining unhybridized $p$ orbital on each carbon then overlaps sideways to form the pi ($\pi$) bond.

The double bond itself comprises one strong sigma ($\sigma$) bond (from head-on $sp^2-sp^2$ overlap) and one weaker pi ($\pi$) bond. A crucial consequence of the pi bond is the restricted rotation around the $C=C$ axis, which is fundamental to understanding geometrical isomerism.

IUPAC nomenclature for alkenes requires identifying the longest carbon chain that contains the double bond, numbering it to assign the double bond the lowest possible position, and replacing the alkane suffix '-ane' with '-ene'.

Substituents are then named and positioned alphabetically. For example, $CH_3-CH=CH_2$ is propene, and $CH_2=C(CH_3)-CH_3$ is 2-methylpropene.

5-Minute Revision

Alkenes are a class of unsaturated hydrocarbons defined by the presence of at least one carbon-carbon double bond ($C=C$). Their general formula is $C_nH_{2n}$. The structural integrity and reactivity of alkenes are dictated by the nature of this double bond.

Structure of the Double Bond: Each carbon atom participating in a double bond undergoes $sp^2$ hybridization. This means one $s$ and two $p$ orbitals combine to form three $sp^2$ hybrid orbitals, which orient themselves in a trigonal planar arrangement, leading to bond angles of approximately $120^\circ$. The remaining unhybridized $p$ orbital on each carbon lies perpendicular to this plane. The double bond itself is a composite of two distinct types of covalent bonds:

1
Sigma ($\sigma$) bond — Formed by the direct, head-on overlap of one $sp^2$ hybrid orbital from each carbon. This is a strong bond, similar to those in alkanes.
2
Pi ($\pi$) bond — Formed by the sideways overlap of the two unhybridized $p$ orbitals, one from each carbon. The electron density of the pi bond is concentrated above and below the plane of the sigma bond. The pi bond is generally weaker and more reactive than the sigma bond.

A key characteristic is restricted rotation around the $C=C$ axis. Unlike single bonds, the pi bond's requirement for parallel $p$-orbital overlap prevents free rotation, which has significant implications for stereochemistry, allowing for geometrical isomers (cis-trans isomers).

IUPAC Nomenclature of Alkenes:

1
Parent Chain — Identify the longest continuous carbon chain that includes the carbon-carbon double bond. This chain determines the parent name (e.g., 'pentene' for a 5-carbon chain).
2
Numbering — Number the parent chain from the end that gives the carbons of the double bond the lowest possible numbers. The position of the double bond is indicated by the number of the first carbon of the double bond. For example, $CH_3-CH_2-CH=CH_2$ is but-1-ene.
3
Suffix — Replace the '-ane' suffix of the corresponding alkane with '-ene'. If there are multiple double bonds, use '-diene', '-triene', etc., indicating all double bond positions (e.g., 1,3-butadiene).
4
Substituents — Name and locate any alkyl groups or other substituents. List them alphabetically before the parent alkene name, with their position numbers.

* Example: For $CH_3-CH(CH_3)-CH=CH_2$, the longest chain with the double bond is 4 carbons (butene). Numbering from the right gives the double bond position 1. A methyl group is at C3. So, the name is 3-Methylbut-1-ene.

This foundational knowledge is critical for understanding alkene reactivity and solving NEET problems related to structure and naming.

Prelims Revision Notes

Alkenes: Nomenclature and Structure of Double Bond

1. Definition & General Formula:

Alkenes — Unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$).
General Formula — $C_nH_{2n}$ (for monounsaturated, acyclic alkenes).
Unsaturation — Presence of $\pi$ bonds or rings reduces hydrogen count compared to alkanes.

2. Structure of the Carbon-Carbon Double Bond ($C=C$):

Hybridization — Each carbon atom involved in the $C=C$ bond is $sp^2$ hybridized.

* One $s$ orbital + two $p$ orbitals $\rightarrow$ three $sp^2$ hybrid orbitals. * One unhybridized $p$ orbital remains on each carbon.

Geometry — Trigonal planar around each $sp^2$ carbon.

* Bond angles: Approximately $120^\circ$. * The entire $C=C$ unit and directly attached atoms lie in a single plane.

Bond Composition — A $C=C$ double bond consists of:

* **One Sigma ($\sigma$) bond**: Formed by head-on (axial) overlap of $sp^2-sp^2$ hybrid orbitals. Stronger bond. * **One Pi ($\pi$) bond**: Formed by sideways (lateral) overlap of the two unhybridized $p$ orbitals. Weaker bond, electron density above and below the internuclear axis.

Bond Length & Strength — $C=C$ bond is shorter ($1.34\,\text{Å}$) and stronger ($614\,\text{kJ/mol}$) than a $C-C$ single bond ($1.54\,\text{Å}$, $348\,\text{kJ/mol}$). The $\pi$ component is weaker than the $\sigma$ component.
Restricted Rotation — The $\pi$ bond prevents free rotation around the $C=C$ axis. This is crucial for geometrical isomerism.

3. IUPAC Nomenclature Rules for Alkenes:

Parent Chain — Select the longest continuous carbon chain that contains the carbon-carbon double bond.
Numbering — Number the parent chain from the end that gives the double bond the lowest possible number. The position of the double bond is indicated by the number of the first carbon of the double bond.
Suffix — Replace '-ane' of the corresponding alkane with '-ene'.
Multiple Double Bonds — Use prefixes like '-diene', '-triene', etc., and indicate all double bond positions (e.g., 1,3-butadiene).
Substituents — Name and locate alkyl groups or other substituents. List them alphabetically with their position numbers before the parent alkene name.

* Example: $CH_3-CH_2-CH=CH_2$ is But-1-ene. * Example: $CH_3-CH=CH-CH_3$ is But-2-ene. * Example: $CH_2=C(CH_3)-CH_2-CH_3$ is 2-Methylbut-1-ene.

Cyclic Alkenes — Prefix 'cyclo-'. Double bond is always between C1 and C2. Number substituents to give them the lowest possible numbers (e.g., cyclohexene, 1-methylcyclopentene).
Common Names — Ethene (ethylene), Propene (propylene), 2-Methylpropene (isobutylene).

Key Points for NEET:

Be able to draw structures from names and name structures.
Accurately count $\sigma$ and $\pi$ bonds.
Identify hybridization and geometry around $C=C$ carbons.
Understand the concept of restricted rotation.

Vyyuha Quick Recall

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).