Question

Draw a structure for cycloheptane. Is the seven-member ring planar? Explain your answer.

Short answer

Cycloheptane is not planar; it adopts a non-planar conformation to minimize strain.

Step-by-step solution

Understanding Cycloheptane

Cycloheptane is a cyclic alkane with the formula \( C_7H_{14} \). It consists of a ring of seven carbon atoms, each bonded to two hydrogen atoms, forming a saturated hydrocarbon.

Drawing the Structure

To visualize cycloheptane, draw a heptagon, each vertex representing a carbon atom. Since each carbon in cycloheptane is bonded to two hydrogens, add each hydrogen atom connected to the sides of the heptagonal ring.

Checking Planarity

Consider the geometry of cycloalkanes. Cycloheptane, with its seven-membered ring, cannot be planar due to angle strain when all atoms lie in a single plane. Ideally, the bond angles in a planar structure would be \(120^\circ\), but the preferred angle for sp3 hybridized carbon is \(109.5^\circ\). Thus, cycloheptane adopts a puckered, non-planar conformation to minimize strain.

Key concepts

Cyclic Alkanes

Cyclic alkanes are hydrocarbons where carbon atoms form a closed ring. Unlike their linear counterparts, cyclic alkanes can take various shapes, depending on the number of carbon atoms involved. This creates a distinct set of chemical and physical properties. What makes them interesting is their ability to form rings of different sizes, from three to many carbon atoms. In the case of cycloheptane, the structure is a seven-membered ring.
Cycloalkanes, like cycloheptane, have all single bonds. This characteristic makes them saturated, meaning they contain the maximum possible number of hydrogen atoms bonded to the carbon skeleton.

Saturated Hydrocarbons

Saturated hydrocarbons are compounds made entirely of carbon and hydrogen atoms connected through single covalent bonds. Since these molecules have a full complement of hydrogen atoms, they are "saturated" in terms of their bonding capacity. Cycloheptane is a perfect example.
Due to this saturation, these hydrocarbons tend to be less reactive than their unsaturated counterparts, such as alkenes and alkynes, which contain double or triple bonds. This stability is often beneficial in the context of industries that rely on chemical stability and predictability.

Bond Angles

Bond angles play a crucial role in determining the shape of a molecule. In organic chemistry, the bond angles in carbon atoms are influenced by the type of hybridization. For sp3 hybridized carbon, like those in cycloheptane, the ideal bond angle is approximately 109.5°. However, when carbon atoms form rings, these angles can be slightly adjusted to remain in comfortable configurations for the atoms.
In smaller rings, such as cyclopropane, the bond angles are much smaller than ideal, leading to significant strain. Cycloheptane, with a seven-membered ring, allows the bond angles to be closer to the ideal, although some adjustments are still necessary.

Ring Strain

Ring strain occurs when a molecule's shape is forced to deviate from its optimal geometry. This is common in cyclic structures where maintaining ideal bond lengths and angles is challenging. Ring strain affects several properties of cyclic alkanes, impacting their stability and reactivity.
For cycloheptane, the seven-carbon ring is large enough to mitigate severe ring strain, unlike smaller rings like cyclopropane or cyclobutane. The molecule adopts a non-planar conformation to better align its bond angles closer to the sp3 ideal, which minimizes the ring strain. This puckered shape helps balance strain, ensuring that cycloheptane is more stable than smaller ring counterparts.