Alkanes & Cycloalkanes Cycloalkanes contain rings of carbon atoms. Chapter 2 Alkanes & Cycloalkanes Section 2 Cycloalkanes Cycloalkanes contain rings of carbon atoms. Ⅰ. Nomenclature Structure Properties

Classification of cycloalkanes Monoscyclic: According to the number of cycles Spirocyclic~ Bicyclic ~* Bridged bicyclic ~ When a bicyclic compound contains 2 rings joined at adjacent carbons, as in bicyclo[4.4.0}decane, it is classified as a fused bicyclic compound,. When the two rings are joined at nonadjacent carbons, as in bicyclo[2.2.1]heptane, the compound is classified as a bridged bicyclic compound. Polycyclic

cyclopropane Small: cyclobutane Common: cyclopentane cyclohexane According to number of carbons on the cycle: Common: cyclopentane cyclohexane Medium: C7~C12 Large: ≥ C13 Since cycloalkanes consist of rings of CH2 units for monocyclo-alkane, they have the general formula (CH2)n, or CnH2n.

Nomenclature 1. Monocycloalkanes 2. Bicyclic bridged hydrocarbons

1. Monocycloalkanes Prefix: cyclo; Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cyclooctane Methylcyclopentane Ethylcyclopropane 1-Isopropyl-4-methylcyclohexane

1,3-Dicyclohexylpropane at the lowest sum 1-Ethyl-3-methylcyclohexane (not 1-methyl-3-ethylcyclohexane) 1-Cyclopropylbutane 1,3-Dicyclohexylpropane 5-Cyclopentyl-1-cyclopropyl-2-methylpentane

cis-1,2-Dimethylcyclopropane trans-1,2-Dimethylcyclopropane Exercises: cis-1,2-Dimethylcyclopropane trans-1,2-Dimethylcyclopropane

2. Bicyclic bridged hydrocarbons A bicycloheptane

[2.2.1] Bicyclo heptane Bicyclo butane [1.1.0] The number of C’s on the shortest one Bicyclo heptane [2.2.1] Bicyclo butane [1.1.0] Note that a fused-ring system always has a 0 as one of the bracketed numbers. The number of C’s on the longest bridge (except for the shared one)

7-Ethyl-1,2,4-trimethylbicyclo[3.3.1]nonane Exercises: 1 2 1 7 3 6 4 5 Bicyclo[3.2.0]heptane 7-Ethyl-1,2,4-trimethylbicyclo[3.3.1]nonane Also name: Decalin Bicyclo[4.4.0]decane

1-Chloro-3,7,7-trimethylbicylco[2.2.1]heptane Q: Give the name of the following . 7 1 2 4 3 1-Chloro-3,7,7-trimethylbicylco[2.2.1]heptane 1-Methylbiicyclo[2.2.0]hexane （1-甲基二环[2.2.0]己烷） 1-Cyclobutyl-1-methylcyclobutane（1-甲基-1-环丁基环丁烷） 1-Methylspiro[3.3]heptane (1-甲基螺[3.3]庚烷)

Structure 1. Ring Strain 2. Conformational isomerism in cyclohexane

1. Ring Strain unstable stable [109.5°- 60°] =49.5° [109.5°- 90°] =19.5°

Table Heats of Combustion of Cycloalkanes(kJ·mol-1) Heat of Combustion /CH2 Group Cyclopropane 3 2091.3 697.1 Cyclobutane 4 2744.3 686.1 Cyclopentane 5 3320.0 664.0 Cyclohexane 6 3951.8 658.6 Cycloheptane 7 4636.7 662.4 Cyclooctane 8 5310.3 663.8 Cyclononane 9 5981.0 664.6 Cyclodecane 10 6635.8 663.6 Cyclopentadecane 15 9884.7 659.0 Unbranched alkane

(1) Cyclopropane bent Banana bond could not lead to ring closure

The large ring strain : ring-opening reactions The large ring strain : ring-opening reactions. Easily react with Cl2, Br2, hydrogen halides (HX), H2SO4, etc, to form addition products at room temperature. Br2 Mechanism: electrophilic addition

(2) Cyclobutane (3) Cyclopentane butterfly conformation “envelope” conformations

Chair form of cyclohexane bond angles of 120o, considerably larger than 109.5o puckered to chair conformation , no angle strain (b) staggered. Cyclohexane can’t undergo ring-opening reactions too.

2. Conformational isomerism in cyclohexane Chair form Boat form

5 1 2 3 4 6 1 2 3 4 5 6 chair form boat form 1 2 4 3 5 6

(torsional strain) eclipsed Q: Which one is more stable? Why? Flagpole Flag H space repulsion eclipsed H’s 183pm 1 4 2 3 5 6 Gunwale 一些原子和基团的范德华半径（pm） H 120；C 150；N 150；O 140； Cl 180 ；CH3 200. Boat form eclipsed (torsional strain) Perspective formula Newman projection

staggered Staggered H’s Chair form Chair form 99.9% Boat form 0.1% 250pm staggered Chair form Chair form 99.9% Boat form 0.1%

Boat conformation: Chair conformation: Characteristics of boat and chair forms: Boat conformation: All adjacent C-H bonds are eclipsed C1-H and C4-H bonds are crowded Chair conformation: All adjacent C-H bonds are staggered Stability: chair’s> boat’s

3. Conformational isomerism of substituted cyclohexane axis equator 3. Conformational isomerism of substituted cyclohexane Equatorial bonds (e bond) and axial bonds (a bond) in chair form e → a a →e Flip up of chair form

How to draw chair conformation of cyclohexane?

Are they equally stable?

equatorial methyl axial methyl strong repulsion 1,3-diaxial interaction (steric strain) - 7.5kJ/mol weak attraction equatorial methyl axial methyl

axial t-butylcyclohexane Conformer of t-butylcyclohexane Steric strain e a equatorial t-butylcyclohexane axial t-butylcyclohexane > 99.9% < 0.1%

have 2 geometric isomers Disubstituted Cyclohexanes There are cis-trans isomers in disubstituted cyclohexane when two substituents are in the different carbon of the ring. 1,4-dimethylcyclohexane have 2 geometric isomers cis- trans-

cis-1,4- and trans-1,4-dimethylcyclohexanes (a,a less stable) (e,e )

cis-1,4-dimethylcyclohexane. (a,e) (a,e)

Exercises: 1. trans-1,3-dimethylcyclohexane (conformer) (a,e) (a,e)

More stable (t-Bt,e;Me,a) (a,e) 2. trans-1-tert-butyl-3-methylcyclo-hexane (conformer) More stable (t-Bt,e;Me,a) (a,e)

Q3 Draw cis-1-tert-butyl-3-methylcyclohexane in its more stable chair conformation. Q4 (a) Draw trans-1,2-dimethylcyclohexane in its more stable chair conformation. Are the methyl groups axial or equatorial? (b) Draw cis-1,2dimethylcyclohexane in its more stable chair conformation. Are the methyl groups axial or equatorial? (c) Which is more stable, cis-1,2-dimethylcyclohexane or trans-1,2-methylcyclohexane? Explain.

Summary: > > > Stability of cycloalkanes: > > > At room temperature, there are many conformations, and 99% conformations are chair’s. Stability: chair’s > boat’s

For the substituted cyclohexane: (a) The longer the distance between the larger groups, the more stable the conformation. (b) For a molecule, the more the large groups on the equatorial bonds, the more stable the conformation.

Q5 Point out the order of stability of the following: Solution: (a) > (b) > (c) > (d)

Q6: Draw the most stable conformation of trans-1-methyl-4-sec-butylcyclohexane. Q7: Draw the most stable conformation of cis-1-methyl-4-sec-butylcyclohexane.

H cis-Decalin trans-Decalin

Properties 1.Free-radical substitution 2.Electrophilic addition In many chemical respects cycloalkanes are similar to the alkanes, but in others they are different: the lower members form addition products with ring fission.

Physical Properties of Cycloalkanes 1. similar to open-chain alkanes 2. higher boiling points than unbranched alkanes 3. much higher melting points than their open-chain counterpart Name formula mp/℃ bp/℃ density（g/cm3） Cyclopropane C3H6 -127.6 -32.7 Cyclobutane C4H8 -80 13 Cyclopentane C5H10 -94 49.5 0.751 Cyclohexane C6H12 6.5 80.8 0.779 Cycloheptane C7H14 -12 117 0.811 Cyclooctane C8H16 13.5 148 0.834

1. Free-radical substitution Chlorine and bromine, in the presence of a catalyst, react with cycloalkanes to form halogenocycloalkanes: excess excess

2. Addition (1) Hydrogenation:

(2) Electrophilic Addition with X2 Cyclopropane reacts with chlorine (Cl2)or bromine at room temperature giving open-ring addition product: room temp CCl4

(3) Electrophilic addition with hydrogen halide 20℃

(Abide by Markovnikov’s rule) When the asymmetric substituted cyclopropane reacts with concentrated HBr or HI, this open addition happens according to following way: (Abide by Markovnikov’s rule)

(Abide by Markovnikov’s rule)

No reaction occur! Q8: Compare the following reactions:

Q9: Differentiate the following compounds. 1. CH3CH2CH2CH2CH3; CH3CH2CH=CH-CH3 2. 、 &

Summary 1.Nomenclature Bicyclic bridged: bicyclo[x.y.z]octane 2.Stability of cycloalkanes: cyclopropane < cyclobutane < cyclopentane ≈ (≥C6) 3. Conformations of Cyclohexane: chair & boat form conformer: 4. Properties: substitution; addition(H2、X2、HX)