Cycloalkanes Open-chain The carbon atoms are attached to one another to form chains Ex: CH 3 -CH 2 -CH 2 -CH 3 n-butane Cyclic compounds the carbon atoms are arranged to form rings called: cyclic compounds, Alicyclic hydrocarbons (aliphatic cyclic hydrocarbons) Ex: Cyclobutane
Alicyclic Hydrocarbons can be classified into: Cycloalkanes Cycloalkenes Cycloalkynes Cycloalkanes Cycloalkanes have molecular formula C n H 2n and contain carbon atoms arranged in a ring.
Nomenclature prefixing cycle- to the name of the corresponding openchain hydrocarbon having the same number of carbon atoms Substituents on the ring are named, and their positions are indicated by numbers, the lowest combination of numbers being used
aliphatic rings are often represented by simple geometric figures: a triangle for cyclopropane, a square for cyclobutane, a pentagon for cyclopentane, a hexagon for cyclohexane
Preparation: Preparation of cycloalkanes from other aliphatic compounds generally involves two stages: A- Cyclization (ring closures): Conversion of some open-chain compound into a compound that contains a ring
B-Modification of a ring compound: Conversion of the cyclic compound thus obtained into the kind of compound that we want 1- Reduction of cycloalkene 2- Reduction of cyclic halide 3- Corey House
Reactions Cycloalkane hydrocarbons undergo the same reactions as their open-chain analogs. 1- Cycloalkanes undergo chiefly free-radical substitution, for example:
2- Reactions of small-ring compounds. Cyclopropane and cyclobutane undergo certain addition reactions. These addition reactions destroy the cyclopropane and cyclobutane ring systems, and yield open-chain products. For example:
Orbital picture of angle strain When carbon is bonded to four other atoms, its bonding orbitals (sp3 orbitals) are directed to the corners of a tetrahedron ; the angle between any pair of orbitals is thus 109.5 o. This means that when carbon is bonded to two other carbon atoms the C-C-C bond angle should be 109.5 o.
In cyclopropane, however, the C-C-C bond angle cannot be 109.5, but instead must be 60. As a result, the carbon atoms cannot be located to permit their sp3 orbitals to point toward each other. There is less overlap and the bond is weaker than the usual carbon-carbon bond. The decrease in stability of a cyclic compound attributed to angle strain
Factors affecting stability of conformations 1- Any atom tends to have bond angles that match those of its bonding orbitals: tetrahedral (109.5) for sp3-hybridized carbon, for example. Any deviations from the "normal" bond angles are accompanied by angle strain.
2- Any pair of tetrahedral carbons attached to each other tend to have their bonds staggered. That is to say, any ethane-like portion of a molecule tends, like ethane, to take up a staggered conformation. Any deviations from the staggered arrangement are accompanied by torsional strain.
Cyclopropane Cyclopropane has a high degree of torsional strain due to large number of eclipsing interactions (eclipsed conformations).
Cyclobutane Cyclobutane has eight H---H eclipsing interactions in a planar structure. Cyclobutane adopts a puckered conformation in order to lower torsional strain.
Cyclopentane The conformation is called the envelope due to its similarity to a mailing envelope.
Cyclohexane The chair conformation of cyclohexane is the most stable. It has no torsional strain as all the C-H bonds are staggered to each other. The bond angle is very close to the ideal all the C-H bonds are staggered to each other. The bond angle is very close to the ideal value.
Boat Conformation: It has no angular strain. However, in addition to the torsional strain resulting from 4 H---H interactions, it also has a flagpole interaction between the hydrogen atoms on 1-and 4-carbon atoms.
Twist Conformation: It is more stable than the boat conformation, but less stable than the chair conformation. The flagpole interactions and torsional strain in the boat conformation are reduced in the twist conformer.