Chapter 3 Alkanes and Cycloalkanes

Two types

◦ Saturated hydrocarbons

◦ Unsaturated hydrocarbons

3.1 Alkanes

Also referred as aliphatic hydrocarbons

General formula: CnH2n+2 (straight chain) and CnH2n (cyclic)

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Alkanes (RH) Alkyl Substituent (R-) Other Substituent (R-)

(“parent” carbon chains)CH4 methane

CH3CH3 ethane CH3CH2CH3 propane

CH3(CH2)2CH3 butaneC5 pentane C8 octaneC6 hexane C9 nonaneC7 heptane C10 decane

(groups attached to parent) CH3 meth (Me) CH2CH3 ethyl (Et) CH2CH2CH3 propyl (Pr)

(groups attached to parent) F fluoro Cl chloro Br bromo I iodo

Structure Presentation

E.g. methane: CH4

All single carbon has four bonding position, completely saturated by the four hydrogen atoms. There is only one possible arrangement of the atoms.

Condensed formula 2D formula 3D formula

CH4

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E.g. Ethane: C2H6

Only two carbon atoms are connected to each other and there are six possible bonding sites. These are filled by the six hydrogen atoms

Condensed formula 2D formula 3D formula

Because single bonds allow rotation, there are number of ways that alkanes can be drawn using slightly different representation.

Constitutional isomers: Compounds that have the same molecular formula but different structural formulas (a different connectivity of their atoms).

• For the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible. There are no constitutional isomers for these molecular formulas.

E.g. How many alkane structures can you draw from C4H10? (*Hint: always start with a straight chain carbon-carbon backbone)

E.g. How many isomers can you draw from C4H9Cl?

* Do cycloalkanes have isomers? YES

E.g. There are two possible ways to make ring using four carbon atoms of C4H8.

E.g How many cyclic constitutional isomers can you draw from C5H10?

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3.2 – 3.4 IUPAC Rules for naming alkanes

1. Find the longest carbon chain (if there is a tie, choose chain with the most substituent). Name parent

2. Number the carbon chain, starting from the end closest to the first substituent

3. Name and number the subtituents (use di, tri, tetra etc.., prefixes for groups that appear more than once).

4. Alphabetize and list substituents before the parent name. Ignore all prefixes other than iso.

E.g.

If there are two substituents on the cycloalkane, number the ring by the beginning with the substituent of lower alphabetical order.

DON’T FORGET to add the pre-fix “cyclo” to the open-chain hydrocarbon (the parent)

If there are three or more substituents, number the ring so as to give the lowest set of numbers, and then list the substituents in alphabetical order.

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3.5 - Classification of Alkyl Halides, Alcohols and Amine

Alkyl Halides Alcohol Amine

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Type of carbons:

Primary (1o) attached to one carbon

Secondary (2o) attached to two carbons

Tertiary (3o) attached to three carbons

The functional group is a halogen (X = F, Cl, Br, I)

The functional group is hydroxyl (-OH)

R-OH

The functional group is a nitrogen atom.

RNH2, R2NH or R3N

Nomenclature

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3.6 Rotation occurs about the carbon-carbon single bond

-rotation about any σ bond without changing the amount of orbital overlapping

- to get different conformational isomers

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E.g Consider the rotation at C2-C3 of butane, CH3CH2CH2CH3

Energy diagram

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3.9 Cycloalkanes: Ring Strain (kcal/mol)

sp3 bond angle can’t be 109.5o (angle strain)

Conformation: Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.

Cyclohexane – no ring strain with two conformations

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In a chair conformation,

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E.g Following is a chair conformation of cyclohexane with carbon atoms numbered 1 through 6.

◦ Draw methyl groups that are equatorial on carbon 1,2 and 4

◦ Draw methyl group that is axial on C2, C3 and equatorial on C6

Conformations and stabilities

If one substituent, then it will prefer conformer in equatorial position

If two substituents on same position, prefer equatorials

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If two position on different position, largest and bulkiest substituent prefer equatorial

3.12-Conformers of disbsubsituted cycloalkanes- cis/trans geometric isomers

The cis isomer has its substituents on the same side of the ring

The trans isomer has its substituent on opposite side of the ring

What about the 1,2-dimethylcyclohexane geometric isomers?

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E.g Which substitution patterns are trans?

1,2 a,a 1,3 a,e 1,4 aa

E.g Determine whether each of the following is a cis isomer or a trans isomers

E.g Draw a chair conformation of the compound below

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