Chem 240: Ch#04- Nomenclature And Conformations Of Alkanes And Cycloalkanes

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Alkanes

Back 1

carbon carbon single bonds

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alkenes:

Back 2

carbon carbon double bonds

Front 3

alkynes:

Back 3

carbon carbon triple bonds

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cycloalkanes:

Back 4

alkanes in which all/ some of the carbon atoms are arranged in a ring.

general formula: CnH2n

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Most alkanes are what type of hybridization?

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sp3

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"unbranched":

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each carbon atom w/in the chain is bonded to no more than two other carbon atoms & that unbranched alkanes contain only primary & secondary carbon atoms.

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Constitutional isomers:

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compounds that have the same molecular formula but have their atoms connected in a different order.

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unbranced alkanes end in...

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-ane

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unbranched alkane with 1 carbon:

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methane

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unbranched alkane with 2 carbons:

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ethane

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unbranched alkane with 3 carbons:

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propane

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unbranched alkane with 4 carbons:

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butane

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unbranched alkane with 5 carbons:

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pentane

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unbranched alkane with 6 carbons:

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hexane

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unbranched alkane with 7 carbons:

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heptane

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unbranched alkane with 8 carbons:

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octane

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unbranched alkane with 9 carbons:

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nonane

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unbranched alkane with 10 carbons:

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decane

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unbranched alkane with 11 carbons:

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undecane

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unbranched alkane with 12 carbons:

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dodecane

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unbranched alkane with 13 carbons:

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tridecane

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unbranched alkane with 14 carbons:

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tetradecane

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unbranched alkane with 15 carbons:

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pentadecane

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unbranched alkane with 16 carbons:

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hexadecane

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unbranched alkane with 17 carbons:

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heptadecane

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unbranched alkane with 18 carbons:

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octadecane

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unbranched alkane with 19 carbons:

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nonadecane

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unbranched alkane with 20 carbons:

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eicosane

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alkyl group:

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end in -yl

when one hydrogen atom from an alkane is removed

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naming branched-chain alkanes:

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isopropyl:

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isobutyl:

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sec-butyl:

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tert-butyl:

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Alkyl Halids:

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When parent chain has both a halo & an alkyl sub. attached to it...

1. # the chain from the end near the 1st sub. regardless of whether it is halo or alkyl

2. if two sub are = dist. from the end of the chain... # from end nearer the sub. that has alphabetical precedence.

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naming alcohols:

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1. choose longest cont. carbon chain which -OH is directly attached.
---- change name of alkane with -ol.

2. # longest cont. carbon chain so as to give the carbon atom bearing the hydroxyl group (-OH) the lowest #.
---- indicate positions of other sub. by using the #s corresponding to their positions along the c-chain as locants.

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alcohols with two hydroxyl groups are named?

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-diols instead of -ol

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cycloalkanes with only one ring are named with?

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cyclo-

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How to name cycloalkanes with 2 substituent?

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number the ring beginning with the substituent fist in the alphabet and number in the direction that gives the next sub. the lower number possible.

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if a single ring system is attached to a single chain with a greater number of carbons how is the compound named?

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cycloalkylalkane

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bicycloalkanes:

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compounds containing two fused/bridged rings.
--- use the name of the alkane corresponding to the total # of carbon atoms on the rings as the parent name.

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bridge head:

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carbon atoms common to both rings

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If substituents are present on a bicycloalkanes:

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- # bridge ring sys beginning @ one bridgehead

- proceeding first along the longest bridge to the other bridgehead

- then along the next longest bridge back to the 1st bridgehead

--- shortest bridge is # last.

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Naming Alkenes:

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alkynes end in...

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-yne

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Alkynide ion:

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when acetylenic # is removed.

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homologous series:

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series of compounds where each member differs from the next member by a constant unit.

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homologues:

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members of a homologous series

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Why is the boiling point of branching alkanes lower?

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b/c of van der waals forces...
- chain branching makes a molecule more compact therefore reducing surface area lowering boiling point.

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Why are alkanes and cycloalkanes almost totally insoluble in water?

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b/c of their very low polarity and their inability to form H-bonds. Liquid alkanes and cycloalkanes are soluble in one another and they generally dissolve in solvents of low polarity.

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Conformations:

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temp. molecular shapes that result from rotation of groups ~ single bonds.

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conformer:

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each possible structure

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conformational analysis:

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analysis of the energy changes associated w/ a molecule undergoing rotation about single bond.

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newman projection:

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sawhorse formula:

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staggered conformation:

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dihedral angle = 180

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eclipse conformation:

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dihedral angle = 0

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Potential energy diagram:

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will spend most of its time in the lowest energy (staggered conf), or in a conf. very close to being staggered. Many times every second, however, it will acquire enough energy through collisions w/ other molecules to surmount the torsional barrier and will rotate through an eclipse conf.

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Torsional barrier:

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small barrier to rotation of the single bond

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torsional strain:

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- factors involved in rotational barriers

- include the orbital considerations

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steric hindrance:

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repulsive interactions

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The 6 important conformers of butane:

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anticonformation:

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- no torsional strain b/c groups are staggered & methyl groups are far apart.

- most stable

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gauche conformation:

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- methyl groups are close enough to each other that the van der waals forces between them are repulsive.

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eclipse conformation:

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- represents energy maxima in the potential energy diagram

- have van der waals repulsions arising from the eclipsed methyl groups & H- atoms.

- greatest energy of all.

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the energy diagram of butane:

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stereoisomer:

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have the same molecular formula & connectivity but different arrangements of atoms in 3D

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Heat of combustion:

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the enthalpy change for the complete oxidation of the compound.

for a hydrocarbon:
complete oxidation means converting it to carbon dioxide & water.

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having more heat of combustion means?

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the isomer is less stable due to having more potential energy

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angle strain:

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the compression of the internal bond angles

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Chair conformation:

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- most stable conformation of cyclohexane ring.

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Energy diagram for cyclohexane:

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Boat conformation of cyclohexane:

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- free of angle strain

- not free of torsional strain

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ring flip of the chair conformation of cyclohexane:

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When the ring flips, all of the bonds that were axial become equatorial and vice versa

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drawing axial and equatorial bonds on a chair conformation of cycloalkane:

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substituents are more stable when they are in the axial or equatorial position on a chair cycloalkane?

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when they are in the equatorial conformation.

- due to generally less repulsive interaction when the groups are equatorial rather than axial.

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Cis:

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up/up
or
down/down

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trans:

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up/down

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Hydrogenation of alkenes:

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hydrogenation of alkynes:

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