Chains that close into rings

Section 1 of 4

The chains you named earlier were all open, with free ends. Join two ends with one more bond, and the chain becomes a ring.

Picture hexane, C₆H₁₄, the alkane with 6 carbons, bent round until its ends meet. Each end carbon already uses all 4 of its hands, so each one lets go of a hydrogen. The two end carbons then bond to each other, and the chain closes into a ring of 6. The new molecule is cyclohexane, C₆H₁₂: the same 6 carbons, with 2 fewer hydrogens.

Hexane's chain closes into the cyclohexane ring Left, the six carbons of hexane drawn as a skeletal chain bent round into an open hexagon, with its two end carbons at the top. Each end carbon has one hydrogen highlighted. An arrow points right, marked 2 H leave, ends join. Right, cyclohexane: a closed hexagon of six carbons, with the new bond between the two old end carbons highlighted. H H hexane, C₆H₁₄ cyclohexane, C₆H₁₂ 2 H leave ends join
Bend hexane round until its ends meet. One hydrogen leaves each end carbon, and the two ends join with a new bond (highlighted) to make cyclohexane.

The name tells you what happened: cyclo- means ring, and hexane says 6 carbons joined by single bonds. Alkanes that form rings are called cycloalkanes.

Hexane, C₆H₁₄Six carbon atoms in a straight chain, joined by single bonds, with fourteen hydrogen atoms around them.HHHHHHHCCCCCCHHHHHHH
Hexane, C₆H₁₄: a chain of 6 carbons. The two end carbons carry 3 hydrogens each.
Cyclohexane, C₆H₁₂Six carbon atoms joined by single bonds in a ring, each carbon also bonded to two hydrogen atoms.CCCCCCHHHHHHHHHHHH
Cyclohexane, C₆H₁₂: a ring of 6 carbons, each carrying 2 hydrogens.

Skeletal formulas from lesson 4 work for rings too. A ring is drawn with one corner for each carbon, so cyclohexane is a hexagon. Every corner is a carbon with two lines to its neighbours, so it carries 2 hidden hydrogens.

Hexane, C₆H₁₄Six carbon atoms in a straight chain, joined by single bonds, with fourteen hydrogen atoms around them.
Hexane as a skeletal formula: a zigzag of 6 carbons.
Cyclohexane, C₆H₁₂Six carbon atoms joined by single bonds in a ring, each carbon also bonded to two hydrogen atoms.
Cyclohexane as a skeletal formula: a hexagon, one carbon at each corner.

Every carbon in a cycloalkane is a corner with 2 hydrogens, so its formula is CnH2n: 2 hydrogens fewer than the open chain, CnH2n+2.

The smallest cycloalkanes
NameFormula
cyclopropaneC₃H₆
cyclobutaneC₄H₈
cyclopentaneC₅H₁₀
cyclohexaneC₆H₁₂

As skeletal formulas they are a triangle, a square, a pentagon and a hexagon. Rings of 5 and 6 carbons are by far the most common, in nature and in the lab. Their bonds can sit close to the 109.5° of methane's tetrahedron. In cyclopropane the bonds are squeezed into a 60° triangle, which makes it much more reactive.

Quick check

Cyclopentane is a ring of 5 carbons joined by single bonds. What is its formula?