What is the primary difference between a sigma ($\sigma$) bond and a pi ($\pi$) bond in terms of orbital overlap?

A $\sigma$ bond is formed by the end-to-end overlap of orbitals along the internuclear axis, while a $\pi$ bond is formed by the sideways overlap of parallel p-orbitals above and below the axis.

How does the reactivity of alkenes compare to alkanes, and why?

Alkenes are much more reactive than alkanes because the $\pi$ bond in the $C=C$ group is a region of high electron density that is easily attacked by electrophiles.

Why is the rotation restricted in a $C=C$ double bond but not in a $C-C$ single bond?

The sideways overlap of p-orbitals in a $\pi$ bond requires the orbitals to remain parallel; rotating the bond would break this overlap and destroy the bond, whereas $\sigma$ bonds have cylindrical symmetry that allows rotation.

What is a common error when naming an alkene with a substituent and a double bond?

Students often number the chain to give the substituent the lowest number, but IUPAC rules state the double bond must take priority and receive the lowest possible locant.

What mistake is often made when applying the general formula $C_nH_{2n}$?

Assuming it applies to all molecules with a double bond; it only applies to non-cyclic alkenes with exactly one double bond. Cyclic alkenes or those with multiple double bonds have different formulas.

What is the error in drawing a $\pi$ bond as two separate bonds?

A $\pi$ bond consists of two lobes of electron density (above and below the plane), but these two lobes together constitute only one single covalent bond containing two electrons.

Define the term 'Unsaturated Hydrocarbon'.

A compound consisting only of carbon and hydrogen that contains at least one carbon-carbon multiple bond (double or triple), meaning it does not contain the maximum possible number of hydrogen atoms.

What is the general molecular formula for the alkene homologous series?

The general formula is $C_nH_{2n}$, where $n$ is the number of carbon atoms in the molecule.

What is an electrophile?

An electrophile is an 'electron-loving' species (usually positively charged or electron-deficient) that is attracted to regions of high electron density, such as the $\pi$ bond in an alkene.

Explain the term 'Functional Group' in the context of alkenes.

The functional group is the $C=C$ double bond, which is the specific part of the molecule that determines its chemical properties and reactivity patterns.

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Describing Alkenes

Summary

Alkenes are a homologous series of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond ( $C=C$ ). This functional group dictates their chemical reactivity and structural geometry, distinguishing them from saturated alkanes through unique bonding arrangements involving sigma and pi orbitals.

1. Definition & Core Concepts

Alkenes are defined as unsaturated hydrocarbons, meaning they contain fewer hydrogen atoms than the maximum possible for the given number of carbon atoms due to the presence of a carbon-carbon double bond ( $C=C$ ).

The $C=C$ double bond serves as the functional group of the molecule, which is the specific atom or group of atoms responsible for the characteristic chemical reactions of the compound.

Because they are hydrocarbons, alkenes consist exclusively of carbon and hydrogen atoms held together by covalent bonds.

The general molecular formula for acyclic alkenes with one double bond is $C_nH_{2n}$ , where $n$ represents the number of carbon atoms in the chain.

2. Bonding Principles: Sigma and Pi Bonds

Diagram showing the sigma bond as a central overlap and the pi bond as two electron clouds above and below the plane of the nuclei.

3. Nomenclature & IUPAC Rules

4. Key Distinctions

5. Exam Strategy & Tips

Describing Alkenes

Summary

1. Definition & Core Concepts

The $C=C$ double bond serves as the functional group of the molecule, which is the specific atom or group of atoms responsible for the characteristic chemical reactions of the compound.

Because they are hydrocarbons, alkenes consist exclusively of carbon and hydrogen atoms held together by covalent bonds.

The general molecular formula for acyclic alkenes with one double bond is $C_nH_{2n}$ , where $n$ represents the number of carbon atoms in the chain.

2. Bonding Principles: Sigma and Pi Bonds

A carbon-carbon double bond is not simply two identical bonds; it consists of one sigma ( $\sigma$ ) bond and one pi ( $\pi$ ) bond, each formed by different types of orbital overlap.

The sigma ( $\sigma$ ) bond is formed by the end-to-end overlap of atomic orbitals (such as s or p orbitals), resulting in high electron density concentrated directly along the internuclear axis between the two carbon atoms.

The pi ( $\pi$ ) bond is formed by the sideways overlap of two adjacent, parallel p-orbitals. This creates two regions of electron density located above and below the plane of the $\sigma$ bond.

The $\pi$ bond is generally weaker than the $\sigma$ bond because the sideways overlap is less effective than end-to-end overlap, making the $\pi$ bond the primary site of chemical reactivity in alkenes.

Diagram showing the sigma bond as a central overlap and the pi bond as two electron clouds above and below the plane of the nuclei.

3. Nomenclature & IUPAC Rules

Alkenes are named using the suffix -ene added to the stem name (e.g., eth-, prop-, but-) which indicates the number of carbon atoms in the longest continuous chain containing the double bond.

For chains with four or more carbons, a number must be used to indicate the position of the double bond, ensuring the lowest possible number is assigned to the first carbon of the $C=C$ unit.

The locant (number) is placed immediately before the '-ene' suffix, separated by hyphens, such as in pent-2-ene for a five-carbon chain where the double bond starts at the second carbon.

If substituents like methyl groups are present, the chain is still numbered to give the double bond priority for the lowest possible number, regardless of the substituent positions.