CHEM 241 Chapter 4 Nomenclature of Alkanes and Cycloalkanes

all memory babyy

bicyclic compounds (bicycloalkane)

two rings are fused on bridgehead carbons

• example was pentalkane and hexalkane in a 3D-like structure

• Recipe

  1. mark bridgeheads

     1. start with bridgeheads that substituents still get lowest number

• (substituents first if necessary)bicyclo[# of C in longest chain ∙ # of C for second longest chain ∙ # of C for shortest path](# of C in bicycloalkane as parent chain)-ane

  • depends on substituents

bridgehead carbons

two carbon atoms where two rings are fused together

Newman projection

visualization of the conformation of a chemial bond from front to back

• look at chemical from certain orientation (eyeball)

• take ALL atoms into account

1. draw front and back

   1. noting that wedges are in front and dashes are behind page

dihedral angle (torsional angle)

the degree of angle separation between atoms that may be altered as the C-C bond rotates

staggered conformation

lowest energy conformation

• most stable conformation

degenerate

the term for when staggered or eclipsed conformations have the same amount of energy

torsional strain/angle strain

the difference in energy (kJ/mol) between staggered and eclipsed conformations of ethane

• for cycloalkanes, an increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5°

  • therefore, 5-membered rings should contain almost no angle strain (in comparison to 4-membered 90° rings or 3-membered 60° rings)

cyclopropane

according to Bayer, the angle strain in this compound is severe

• some strain may be alleviated if orbitals bend outward, but may have increase in energy associated w/ inefficient overlap of orbitals

cyclobutane

has less angle strain than cyclopropane, but has more torsional strain due to 4 sets of eclipsing H’s

• to alleviate torsional strain, can adopt a slightly puckered conformation w/o gaining too much angle strain

cyclopentane

has an envelope conformation as well as less angle strain than cyclobutane or cyclopropane

• reduces much of its torsional strain by adopting envelope conformation

• still experiences some strain, but has less total strain than cyclopropane or cyclobutane

cyclohexane

the most important conformation that can adopt 2 types of conformation:

1. chair conformation = most stable conformation, all substituents would be staggered

2. boat conformation = still may experience torsional strain due to flagpole interactions (which may lead to twist boat conformation)

• bond angles fairly close to 109.5° and possess little angle strain

1. drawing both chair conformations at once, draw the double line “V’s” and the spikes

2. same rules apply when assigning locants to 2D structure

   1. substituents get lowest locants

3. Number the chair conformations

   1. starting at point on the right, go clockwise around the chair

4. using the 2D structure as a guide (wedge = up, dash = down) draw the substituents as axial or equatorial

   1. pay attention to orientation of locant on chair

5. if asked for most stable chair conformation, will be structure with more equatorial substituents (less axial)

   1. due to 1,3-diaxial interactions causing the chair conformation to be higher in energy when substituent is in the axial position

Recipe for drawing chair conformations

anti-conformation

the conformation with a dihedral angle of 180°

• represents the lowest energy conformation

• in butane, would be the conformation when the methyl groups are farthest apart

gauche interaction

an unfavorable energy conformation (steric interaction) where in butane, the methyl groups in staggered conformations are next to each other, repel each other, which cause an increase of energy

eclipsed conformation

highest energy conformation

• most unstable conformation

axial position

substituents either straight up or down on chair conformation

equatorial position

substituents on sideway on chair conformation

equatorial

• this is because substituents in the axial position may experience steric interactions called flagpole interactions

The most stable chair conformation will contain more substituents that are in the ____________position.

alkanes

hydrocarbons that lack π-bonds

cycloalkanes

alkanes that contain a ring (if the longest C-chain is located as a ring)

substituents

simple or branched alkyl groups that will be apart from the parent chain

locant

the numbering of C along the parent chain to identify the “location” of each substituent

1. Id. the parent chain

   1. choose the longest chain

   2. choose the chain with the greater number of substituents

2. Id. and name the substituents

3. Number the parent chain and assign a locant to each substituent

   1. give the 1st substituent with the lowest possible number, then where the second substituent has the lower number

4. Arrange the substituents alphabetically

   1. place locants in front of each substituent

      1. substituent prefixes are ignored when alphabetizing

• ex. locant-substituent(-locant 2-substituent 2)parentname

Recipe for assigning alkane names

isopropyl (1-methylethyl)

an alkyl group bearing 3-C atoms that can only be branched one way

sec-butyl (1-methylpropyl)

4-C substituent with one bond to the parent chain and the C atoms are in a linear arrangement

isobutyl (2-methylpropyl)

4-C substituent with a stickman legs

tert-butyl (1, 1-dimethylethyl)

4-C substituent with 3 tree branches

isopentyl (3-methylbutyl)

5-C substituent with stickman legs

neopentyl (2,2-dimethylpropyl)

5-C substituent with 3 tree branches

cis

used to signify that the two groups (substituents) are on the same side of the ring

trans

used to signify that the two groups (substituents) are on opposite sides of the ring

decalin

a bicyclic system composed of 2 fused 6-member rings

• cis-decalin and trans-decalin are stereoisometric (different compounds with different physical properties

cis-decalin

both substituents face the same side (both up or both down)

trans-decalin

both substituents face different directions (1 up and 1 down)

The less stable compound would be expected to have a larger heat of combustion. Straight chain alkanes would have greater heat given off during the reaction due to them being less stable (larger).

For heat of combustion, would a more stable or less stable compound be expected to have a larger heat of combustion?

• branched alkanes are lower in energy (more stable) than straight chain alkanes