Chapter 4: Alkanes and Stereochemistry

alkanes

family of hydrocarbons, single bonded C-H

isomers

compounds that have the same # and kinds of atoms but differ in atomic arrangement

constitutional isomers

isomers that have their atoms connected in a different order

• different carbon skeletons, functional groups or position of functional groups 

primary carbon

bonded to one other carbon

secondary carbon

bonded to 2 other carbons

tertiary carbon

bonded to 3 other carbons

quaternary carbon

bonded to 4 other carbons

alkane properties

• paraffins: little affinity for other substances 

• reacts w/oxygen, halogens and few more 

• reactions w/oxygen occur during combustion 

stereochemistry

3D aspects of molecules —> 3D structure determines properties and biological behavior of molecules

conformations

3D shape of molecule (rotation around single bonds are frozen)

sawhorse representation

views carbon-carbon bond from angle

• shows all C-H bonds 

newman projects

views carbon-carbon bond directly end-on

• represents 2 carbons by a circle 

• staggers and eclipsed

staggered

• lowest energy, most stable

• all 6 C-H bonds are as far away as possible

eclipsed

• highest energy, least stable

• 6 C-H bonds are as close together as possible

• greatest steric and torsional strain

anti-conformation

• lowest energy conformation, 2 largest substituents are 180 degrees apart

• multiple carbons

• type of staggered confirmation

gauche conformation

• 2 methyl (CH3) groups lie 60 degrees apart

• higher energy than anti-conformation 

• steric strain 

• type of staggered confirmation 

steric strain

repulsive interaction in molecule when 2 groups are closer together than their atomic radii allow (between non bonded atoms in close proximity)

• anti-conformation vs gauche conformations

cis-trans isomerism

• cycloalkanes have top and bottom face = isomerism 

stereoisomers

compounds with atoms connected in the same order, but differ in 3D orientation

• cis or trans 

constitutional isomers

atoms are connected in a different order

ring strain + stability of cycloalkanes

cyclic molecules can assume nonplanar conformations to minimize angle + torsional strain by ring-puckering 

• larger rings have more possible conformations 

• larger than 3 atoms 

• measure strain measuring heat of combustion energy

angle strain

expansion or compression of bond angles away from most stable

torsional strain

eclipsing of bonds on neighboring atoms

• staggered vs eclipsed conformations

cyclopropane conformation

• planar, 3-membered ring

• C-C-C bond angles of 60 degrees

• sp3 bonds are bent + weakened

• all C-H bonds are eclipsed

cyclobutane 

• less angle strain but more torsional strain (larger number of ring hydrogens)

• bend increases angle strain but decreases torsional strain 

• not quite eclipsed 

cyclopentane

• no angle strain but high torsional strain (planar)

• non-planar reduces torsional strain

cyclohexane chair conformation

• ring is free of angle and torsional strain

• alternating atoms in common plane w/tetrahedral angles

• chair conformation 

• bonds are staggered

cyclohexane boat conformation

• less stable than boat due to steric and torsional strain 

• 4 eclipsed H-pairs on C 2, 3, 5 and 6 that produce torsional strain

• bonds are staggered + eclipsed

axial

point straight up or down

equatorial

point outwards

• more stable

flipping from chair to boat conformation

hydrogens change from equatorial to axial

1,2 cis

a,e or e,a

1,2 trans

a,a or e, e

1,4 cis

same as 1,2 cis

1,3 cis

opposite of 1,2 cis (a,a or e, e)