Bài giảng hoá học hữu cơ cycloalkanes

Ring strain The carbon atoms in cycloalkanes are sp3 hybridized and a deviation from the ideal tetrahedral bond angles of 109.50  This causes an increase in potential energy and an ov

Nomenclature

 Carbocycles are organic molecules that

contain one or more rings

 Cycloalkanes are simplest cyclic molecules which have molecular formulas CnH2n

 Cycloalkanes can be drawn as regular

polygons using line-angle representations

 Cycloalkanes are named after their

corresponding linear alkanes with the

prefix -cyclo .

 When more than one substituent is

present, their positions on the ring are

numbered in such a way that substituents receive the lowest possible numberings

Nomenclature of substituted cycloalkanes

 When the two substituents point to

opposite faces, they are trans.

 cis-trans Isomers in cycloalkanes are stereoisomers

Ring strain

 The carbon atoms in cycloalkanes are sp3

hybridized and a deviation from the ideal tetrahedral bond angles of 109.50

 This causes an increase in potential

energy and an overall destabilizing effect

 Eclipsing of hydrogen atoms is an

important destabilizing effect, as well

Conclusion:

The strain energy of a cycloalkane is caused by the compound's geometry.

Cyclopropane

 In Cyclopropane the carbon atoms form a triangle having C-C-C bond angles of 600

 Big deviation from

the ideal tetrahedral

angles of 109.50

 There are also three pairs of eclipsed Hs.

 Ring strain is highest in cyclopropane,

around 120 kJ/mol

 “Puckering" reduces the eclipsing

interactions between H atoms

 Its ring strain is slightly less than in the case of cyclopropane,

 it’s at around 110 kJ/mol

 Theoreticaly, the C-C-C bond angles of planar cyclopentane would be 1080;

 very close to the tetrahedral angle

 Actual cyclopentane molecules are

puckered, but the bond angles changes slightly so that angle strain is relatively small

 The eclipsing interactions are also reduced, leaving a ring strain of about 25 kJ/mol.

to be the most stable

 In the chair conformations all

carbon-carbon bond angles are 109.50

 and are thereby free of angle strain

 This conformation is free of torsional

strain as well

 When the conformation is viewed along

any carbon-carbon bond,

 the bonds are seen to be perfectly

staggered  the H-atoms at opposite corners of the cyclohexane ring are maximally separated

 The boat conformation.

 It’s also free of angle strain, but

 it’s not free of torsional strain.

 When the conformation is viewed down carbon-carbon bond axes along either

side, the C-H bonds at those carbon atoms are found to be eclipsed

 Additionally, two of the hydrogen atoms

on C1 and C4 are close enough to each

other to cause van der Waal repulsion,

 this effect is called the “flagpole”

interaction

 Torsional strain and flagpole interaction cause the boat conformation to have

considerably higher energy than the chair conformation

 More than 99% of the molecule are

estimated to be in a chair conformation at any given moment

 The chair conformation of cyclohexane has two kinds of positions for hydrogens

on the ring:

 axial positions that are perpendicular

to the ring (parallel to the ring axis) and

 equatorial positions that are in the

rough plane of the ring (around the ring equator)

 Each carbon atom has one axial and one equatorial hydrogen.

 There are 6 axial hydrogens and 6

equatorial hydrogens in alternating

arrangement

axial

equatorial

 Two hydrogens on the same side of a ring

are always cis, regardless of whether

they’re axial or equatorial and regardless

of whether they’re adjacent.

 Similarly, two hydrogens on opposite

sides of the ring

are always trans

trans

trans cis

 Different chair cyclohexane conformations readily interconvert, resulting in the

exchange of axial and equatorial positions

 This interconversion of chair conformations usually referred to as a ring-flip show:

Move this carbon down

Move this carbon up

Ring-flip

Methylcyclohexane

 Axial and equatorial methylcyclohexane interconvert rapidly, but they aren’t

equally stable.

 The equatorial conformation is more

stable (95% ) than the axial conformation

by 7.6 kJ/mol (1.8 kcal/mol)

 The energy difference is due to an

unfavorable steric interaction that occurs between the axial methyl group (C1) and the axial hydrogen atoms on C3 and C5

H

H

1 3

5

H

 For other monosubstituted cyclohexane: a substituent is always more stable in a

equatorial position than in an axial position

 The steric strain increases as the size of the axial substituent group increases

Preparation

 Cycloalkanes can be formed in a Diels-Alder

reaction followed by a catalytic

hydrogenation

 Industrially, cyclohexane is made by catalytic hydrogenation of benzene in the presence of a nickel catalyst at about 200 0 C and 20-40 bar:

+ 6H Ni

 The small cycloalkanes have a lower

stability; they react similarly to alkenes

 Cyclopropane reacts with ring-opening:

Bicyclic alkanes

 One of the most important bicyclic system

is bicyclo[4.4.0]decane, a common name,

 Bicyclo[2,2,1]heptane