Properties of common rings (C ≥ 5)
Common cycloalkanes, whose structure contains five or more carbon atoms, exhibit high chemical stability. In their behavior and reactivity, they are practically identical to non-cyclic (aliphatic) alkanes.
These compounds are most characteristic of free-radical substitution ($S_R$) reactions. In these processes, the carbon cyclic backbone remains intact, and only the hydrogen atoms attached to the carbon chain undergo substitution.
The stability issue of small rings
Small rings traditionally include cyclopropane ($C_3H_6$) and cyclobutane ($C_4H_8$). The main distinguishing feature of these molecules is their pronounced instability compared to common rings.
The root of this instability lies in the spatial geometry of the molecule. Carbon atoms in cycloalkanes are in a state of $sp^3$-hybridization, for which the ideal (tetrahedral) angle between $C-C-C$ bonds is strictly $109.5^\circ$. However, in small rings, the actual bond angles deviate significantly from this normal value.
This geometric mismatch leads to a major consequence: a sharp decrease in chemical bond energy. A so-called "ring strain" arises in the molecular structure, making the compound chemically vulnerable and prone to ring-opening reactions.
Chemical reactions of small rings
Due to strong internal strain, small rings constantly tend to open. Therefore, unlike common rings that undergo substitution reactions, small rings are prone to addition reactions.
This process is accompanied by the cleavage of the carbon ring, and new atoms add to the ends of the resulting open chain.
Reaction examples:
- Action of halogens (using cyclopropane as an example): When reacting with bromine, the cyclopropane ring opens. Bromine adds to the terminal carbon atoms, resulting in the formation of linear 1,3-dibromopropane.
$$\triangle + Br_2 \rightarrow BrCH_2CH_2CH_2Br$$
- Hydrogenation (using cyclobutane as an example): When hydrogen ($H_2$) is added to cyclobutane, one of the ring bonds is cleaved, and the molecule turns into a common non-cyclic alkane — n-butane.
$$\square + H_2 \rightarrow CH_3CH_2CH_2CH_3$$
Methods for producing cycloalkanes
One of the methods for synthesizing cycloalkanes is obtaining them from dihaloalkanes. A characteristic example is the use of 1,5-dibromopentane.
The process is an intramolecular cyclization reaction that proceeds via the Wurtz reaction mechanism. Active metals must be used for the successful abstraction of halogen atoms and the closure of the carbon chain into a stable ring. Sodium (Na), zinc (Zn), or magnesium (Mg) are traditionally used as such reagents.
Briefly: answers to topical questions
What are the stages of the free-radical substitution ($S_R$) mechanism in common cycloalkanes?
The stages of the free-radical substitution mechanism in common cycloalkanes are similar to those of non-cyclic alkanes and include three consecutive steps.
- Chain initiation — dissociation of a halogen molecule into two free radicals under the action of light.
- Chain propagation — a halogen radical abstracts a hydrogen atom from the hydrocarbon, forming an alkyl radical, which then reacts with a halogen molecule to form a substitution product and a new radical.
- Chain termination — the process ends when free radicals collide with each other to form a stable molecule.
What methods exist for producing cycloalkanes other than intramolecular cyclization?
In addition to intramolecular cyclization, cycloalkanes can be obtained by hydrogenation of aromatic hydrocarbons and catalytic synthesis.
- Hydrogenation of benzene — a hydrogen addition reaction upon heating with a nickel catalyst, yielding cyclohexane.
- Synthesis on cobalt catalysts — production of a mixture of alkanes and cycloalkanes from carbon monoxide(II) and hydrogen (syngas).
To remember chemical behavior: "Large rings are lazy—they only substitute. Small rings are strained—they burst and add."
What else to study on this topic
- Mechanism of free-radical substitution ($S_R$) reactions in common cycloalkanes
- Features of $sp^3$-hybridization and the formation of the $109.5^\circ$ angle
- Effect of active metals (Na, Zn, Mg) on the course of the Wurtz reaction
- Energetic causes of "strain" formation in small rings
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Cycloalkanes" and 3,009 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 99 video tutorials for the subject
Other topics in "Chapter 15. Hydrocarbons"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.