Question

Indicate whether each statement is true or false. (a) Butane contains carbons that are \(s p^{2}\) hybridized. (b) Cyclohexane is another name for benzene. (c) The isopropyl group contains three \(s p^{3}\) -hybridized carbons. (d) Olefin is another name for alkyne.

Short answer

(a) False, (b) False, (c) True, (d) False

Step-by-step solution

Statement (a) Butane contains carbons that are \(sp^2\) hybridized.

Butane is an alkane with the chemical formula C4H10. In alkanes, all carbon atoms are \(sp^3\) hybridized, as they are tetrahedrally bonded with four single bonds (either to other carbon atoms or to hydrogen atoms). There are no \(sp^2\) hybridized carbons in butane, as these are only present in compounds with double bonds, such as alkenes. Therefore, the statement is false.

Statement (b) Cyclohexane is another name for benzene.

Cyclohexane and benzene are two distinct chemical compounds. Cyclohexane is a cycloalkane with the chemical formula C6H12, characterized by a six-membered ring of carbon atoms, each bonded to two hydrogen atoms. Benzene, on the other hand, is an aromatic compound with the chemical formula C6H6, characterized by a six-membered ring of carbon atoms, with alternating single and double bonds, and each carbon atom bonded to one hydrogen atom. Therefore, the statement is false.

Statement (c) The isopropyl group contains three \(sp^3\)-hybridized carbons.

The isopropyl group (also known as a 2-propyl or isopropylidene group) is a hydrocarbon group with the chemical structure \(\mathrm{CH_3CH(CH_3)−}\). It consists of a central carbon atom (the ipso carbon) bonded to a hydrogen atom and two methyl groups. In the isopropyl group, all carbon atoms are \(sp^3\) hybridized, with their tetrahedral arrangement of single bonds. Therefore, the statement is true.

Statement (d) Olefin is another name for alkyne.

Olefin is an older synonym for alkene, not alkyne. Alkenes are hydrocarbons that include a carbon-carbon double bond. Olefins thus contain \(sp^2\) hybridized carbons. Alkynes, on the other hand, are a distinct class of hydrocarbons that contain a carbon-carbon triple bond, with the bonded carbons being \(sp\) hybridized. Therefore, the statement is false.

Key concepts

Hybridization

Understanding hybridization is essential for grasping how carbon atoms form bonds in organic molecules. Hybridization refers to the mixing of atomic orbitals in an atom to form new hybrid orbitals. These new orbitals are suitable for pairing to form chemical bonds.
In organic chemistry, three primary types of hybridization are commonly seen:

• sp³ Hybridization: This form involves the mixing of one s orbital with three p orbitals, resulting in four equivalent sp³ hybrid orbitals. Carbon atoms with sp³ hybridization have a tetrahedral geometry and form single bonds. Alkanes, like butane, exclusively feature sp³ hybridized carbons.
• sp² Hybridization: Here, one s orbital mixes with two p orbitals to produce three sp² hybrid orbitals. This hybridization allows for planar geometry with one double bond, common in alkenes.
• sp Hybridization: This results from the mixing of one s and one p orbital. It forms two sp hybrid orbitals, allowing for linear geometries consistent with triple bonding, as seen in alkynes.

Grasping these hybridization types allows you to predict molecular shapes and bond types.

Alkanes

Alkanes are the simplest family of hydrocarbons, consisting entirely of single bonds between carbon and hydrogen atoms. They have the general formula C_nH_2n+2, and each carbon atom in an alkane is sp³ hybridized, leading to a tetrahedral geometry.
Alkanes are saturated compounds, meaning they contain the maximum number of hydrogen atoms per carbon atom. This saturation makes them relatively stable and less reactive compared to other hydrocarbons. Key characteristics of alkanes include:

• Non-polarity: Due to their symmetric shape and lack of a functional group, alkanes are non-polar and exhibit weak van der Waals forces.
• Combustion: Alkanes burn in the presence of oxygen to produce carbon dioxide and water, making them valuable as fuels.
• Low Reactivity: Their single-bonded structures make alkanes less reactive, behaving primarily through substitution in halogenation reactions.

Butane, an example of an alkane, has four carbon atoms, each bonded to hydrogen atoms and displaying sp³ hybridization.

Alkenes

Alkenes are hydrocarbons characterized by at least one carbon-carbon double bond. This double bond is the defining feature of alkenes and contributes to their increased reactivity compared to alkanes. Alkenes are often referred to by another name, olefins.
The general formula for alkenes is C_nH_2n, reflecting the nature of the double bonds. Key aspects of alkenes include:

• sp² Hybridization: The carbons involved in the double bond are sp² hybridized, leading to a planar structure around the double bond area.
• Reactivity: The presence of a double bond makes alkenes more reactive than alkanes. They readily participate in addition reactions, where different atoms or molecules add across the double bond.
• Applications: Alkenes serve as important building blocks in the synthesis of polymers, such as polyethylene, through polymerization reactions.

Analyzing alkenes requires careful consideration of the double bond, influencing both the chemical and physical properties of these compounds.

Aromatic Compounds

Aromatic compounds, commonly referred to as aromatics, are a special class of hydrocarbons characterized by their stability and unique bonding patterns. Benzene is the prototypical aromatic compound, with a distinct ring structure and a chemical formula of C₆H₆.
Aromatic compounds are not merely cyclic compounds; they need to follow specific criteria known as Huckel's rule, which requires the molecule to have

• Planar structure, allowing for effective overlap of orbitals
• Conjugated systems with alternating single and double bonds, providing delocalization of electrons
• A number of \((4n+2)\) \(\pi\) electrons, where n is a positive integer

Benzene is the model aromatic compound featuring a hexagonal ring with alternating double bonds, which are actually equal due to electron delocalization across the ring. This delocalization imparts extraordinary stability to aromatic compounds—a property exploited in countless industrial applications. Understanding aromatics is crucial in the study of organic chemistry due to their broad presence in pharmaceuticals, dyes, and polymers.