Alicyclic Chemistry (Cycloalkanes) CHM1206 University of Guyana

What is "Alicyclic Chemistry"?

The chemistry of ring-shaped compounds.

What are Cycloalkanes?

alkanes in which the carbon atoms are arranged in a ring.

What is the general formula for a cycloalkane? Why is it Cn​H2n​ instead of Cn​H2n+2​?

Cn​H2n​. It has two fewer hydrogen atoms compared to acyclic alkanes (Cn​H2n+2​) because forming a ring requires the loss of two hydrogen atoms.

How do you name simple (unsubstituted) cycloalkanes?

add the prefix "cyclo-" to the name of the corresponding acyclic alkane with the same number of carbons. Eg: Cyclopentane

How are cycloalkane structures typically represented? What does each vertex mean?

As regular polygons. Each vertex represents a carbon atom

What is Rule 1 for naming substituted cycloalkanes regarding the parent name?

Decide whether the cyclic or acyclic portion contains more carbons. This determines the base (parent) name.

When is the ring named as the parent in a substituted cycloalkane?

If the number of carbon atoms in the ring is equal to or greater than the number in the substituent (alkyl chain). (e.g., methylcyclohexane).

When is the acyclic chain named as the parent in a substituted cycloalkane?

If the number of carbon atoms in the acyclic chain is greater than the number in the ring. (The cyclic part then becomes a cycloalkyl substituent, e.g., cyclopropylpentane).

What is Rule 2 for naming substituted cycloalkanes regarding numbering?

: Carbons are numbered to give the lowest possible numbers for all substituted carbons. Start numbering at the most substituted carbon, and then proceed in the direction that gives the lowest numbers to the other substituents.

For a cycloalkane with only one substituent, do you need to number the carbon it's on?

No, by definition, that carbon is carbon #1. (e.g., methylcyclopropane, not 1-methylcyclopropane).

How do boiling and melting points of cycloalkanes generally change with increasing molecular mass?

They increase with increasing molecular mass.

How do boiling and melting points of cycloalkanes compare to comparable acyclic alkanes (same number of carbons)? Why?

They are higher. This is due to their more rigid structure, which allows for greater intermolecular interactions and more molecular packing.

How does the density of cycloalkanes compare to comparable acyclic alkanes? Why?

They have higher density. This is because their rigid structure allows molecules to 'pack' more effectively than corresponding alkanes, thereby increasing mass per unit volume.

Do open-chain alkanes undergo free rotation about their C-C bonds?

Yes

Do alkenes undergo free rotation about their C=C bonds? What's the consequence?

No. This restriction leads to geometric isomerism (cis and trans isomers).

Do cycloalkanes undergo free rotation about their C-C bonds within the ring? What's the consequence?

No. Their ring structure prevents free rotation, meaning substituents are fixed. This leads to geometric isomerism (cis and trans isomers).

Define the "cis" isomer in substituted cycloalkanes.

If two non-hydrogen substituents are on the same side of the ring.

Define the "trans" isomer in substituted cycloalkanes.

If two non-hydrogen substituents are on opposite sides of the ring