Hydrocarbons

Organic structures that contain only carbon and hydrogen

Saturated – a compound is termed “saturated” if it has the maximum hybridization (sp3) at each carbon

Therefore: no double or triple bonds

A saturated carbon species is termed an ALKANE

The compound has a root name indicating the number of carbons and the –ane suffix

CH3-CH3 ethane

An ALKENE has a carbon-carbon double bond

All three structures represent ethylene (or ethene)

An ALKYNE has a carbon-carbon triple bond

All three structures represent acetylene (or ethyne)

Straight Chain Alkanes

The alkanes are named according to the number of carbon atoms in the chain

Ends with an –ane suffix

Root name # of carbons (n) H-(CH2)n-H

Meth- 1 Eth- 2 Prop- 3 But- 4 Pent- 5 Hex- 6 Hept- 7 Oct- 8 Non- 9 Dec- 10

All alkanes have the empirical formula CnH(2n+2)

Origin of Naming for Alkanes

C1 through C4 are result of common names for carbon chains, C5 through C10 are named due to the Greek word for their root

(an 8 sided circle for example is an octagon – OCT represents 8)

Meth - means wine or spirit in Greek, yl – means wood or matter in Greek

Therefore methyl alcohol (which has one carbon) means a spirit from wood Methanol is obtained from distillation of wood (sometimes called wood alcohol)

METH is thus kept for a 1 carbon chain, yl is kept to mean a carbon group and is used for any carbon substituent

(methyl, ethyl, propyl, etc.)

ETH root comes from Greek word ether (to shine) Shine → sky → colorless liquid

Ether (also called diethyl ether) is a colorless liquid and it has two 2-carbon chains a two carbon chain is ETH

PROP common name is a result of the three carbon chain acid called propionic acid

Protos (Greek for first), pion (Greek for fat) Propionic acid thus literally means “first fat”

1 carbon acid is formic acid (from ants) 2 carbon acid is acetic acid (from vinegar)

Both formic acid and acetic acid are soluble in water due to the low carbon content, Propionic acid is thus the smallest acid chain that is not soluble in water but soluble in

organic solvents (thus first fat – fatty acids are long chain carboxylic acids)

BUT comes from the common name for a 4 carbon carboxylic acid (butyric acid) Butyric acid is the cause for the smell in rancid butter

(where BUT comes from the word for butter)

Hofmann’s attempt for Systematic Hydrocarbon Nomenclature (1866)

Attempted to use a systematic name by naming all possible structures with 4 carbons

Quartane C4H10 Quartyl C4H9 Quartene C4H8 Quartenyl C4H7 Quartine C4H6 Quartinyl C4H5 Quartone C4H4 Quartonyl C4H3 Quartune C4H2 Quartunyl C4H1

Wanted to use Quart from the Latin for 4 – this method was not embraced and BUT has remained

IUPAC Nomenclature

Procedure for naming carbon chains containing branches or substituents (non-straight chain)

1)  Find the longest continuous carbon chain in the structure -this determines the root name

2) Any carbon not on this continuous chain is a substituent (appendage)

3) Number the main chain starting from the end closest to the first substituent

4)  The substituents are still named according to the number of carbons (the suffix for a substituent is –yl instead of –ane)

-CH3 methyl -CH2CH3 ethyl

5) Place all substituent names before the root name in alphabetical order

6) The substituent must be numbered to indicate the point of attachment to the main chain

7) Group multiple substituents of the same kind together and label di-, tri-, etc.

8)  When alphabetizing, the prefixes di-, tri, n-, t- are ignored (the only prefix used for alphabetizing is iso-, explained in common names)

9)  With a ring compound the number of carbons in the ring determines the root name with a cyclo- prefix

10)  Halogens are named as substituents with an o suffix e.g. fluoro-, chloro-, bromo- or iodo-

Common Names

Many alkyl substituents have common names

Consider propyl

There are two ways an alkyl appendage with three carbons can be attached

Any straight chain appendage has the n- prefix (for normal)

CH3CH2CH2- n-propyl

This distinguishes the straight chain compound from the other isomer

Isopropyl (1-methylethyl) using IUPAC

Use iso prefix (short for isomer)

With larger alkyl substituents, the more possibilities for isomers exist

Consider butyl

H3CH2CH2CH2C

CH

H3C

H3CH2C

CH2CH

H3C

H3C

C

CH3

H3C

CH3

n-butyl

isobutyl

secbutyl (s-butyl)

tertbutyl (t-butyl)

The sec- and tert- prefixes for common names are based upon degree of substitution A carbon bonded to three other carbons is called a tertiary carbon

A carbon bonded to two other carbons is called a secondary carbon

A carbon bonded to one other carbon is a primary carbon

To name substituents, only consider the bonding pattern of the carbon directly bond to the main chain, and then consider how many other carbons are bonded to that carbon to obtain

tert- or sec- names

e.g. tertbutyl

CH

H3C

H3CH2C

secondary carbon(2˚)

C

CH3

H3C

CH3tertiary carbon(3˚)

CH2CH

H3C

H3Cprimary carbon

(1˚)

e.g. secbutyl

e.g. both n-butyl and isobutyl

Complex Alkyl Groups

As the alkyl substituents become more complicated (e.g. more branching) the same IUPAC rules are followed and the name for the whole appendage is placed in parenthesis

The root is the cyclooctane ring (usually the ring is used as a root although if the number of carbons in the substituent become

larger then the ring could be named as a substituent)

ethyl substituent 1,1,3-trimethylbutyl substituent

(with substituents need to count from the carbon at the attachment to root and find longest chain)

After alphabetizing: 1-ethyl-3-(1,1,3-trimethylbutyl)cyclooctane

Attractive Forces in Alkanes

- Type of electron correlation between molecules determine the physical properties

Coulombic attraction dipole-dipole van der Waals forces (London dispersion)

Conformational Analysis of Alkanes

- Physical properties of molecules are determined by intermolecular forces (forces between molecules)

- The internal structure of a given molecule can affect the energy due to sterics (intramolecular interactions)

Conformer: different arrangements in space resulting from the rotation of bonds (bonds are not broken when interconverting between conformers)

Consider Methane

H

H HH

No conformers possible; methane has a given energy value that does not change (any rotation about the equivalent C-H σ bonds yields the same structure

in three-dimensions)

*this is not the case with any higher hydrocarbon homologue

Conformational Analysis of Ethane

Structures have different energy due to different arrangements of space (hydrogens have different spatial arrangements in different conformers)

Newman Projections

- Convenient way to view conformational analysis

To Draw Newman Projections

1) Determine which bond is being considered

2) Determine which atom is front atom of bond being considered

3)  The substituents attached to the front atom are drawn to a point, the substituents attached to the back atom are drawn to a circle

4) The relative angles and orientation of the substituents are maintained

Newman projections of ethane conformations

Newman projections demonstrate energetic and spatial interactions of conformers

Eclipsed conformations are higher in energy

One cause is the sterics As the substituents that are eclipsed become larger, the energy of the conformer raises

Consider the space filling area of atoms

Conformational Energy Diagram for Propane

Different Types of Interactions Arise with Larger Carbon Structures

Consider n-Butane viewing down the C2-C3 carbon-carbon bond

Rings (Cycloalkanes)

Due to the ring the σ bonds cannot rotate 360˚ as in alkanes

Do not have the same conformational analysis as with other alkanes

Therefore rings adopt a certain preferred geometry

Rings Strain for Simple Cycloalkanes

Ring size cycloalkane Total ring strain

(Kcal/mol)

Ring strain per CH2

(Kcal/mol) 3 cyclopropane 27.6 9.2 4 cyclobutane 26.4 6.6 5 cyclopentane 6.5 1.3 6 cyclohexane 0 0 7 cycloheptane 6.3 0.9 8 cyclooctane 9.6 1.2

Small rings have large strain Cyclohexane has the least amount of strain

Conformation of Cyclopropane

All three carbon atoms must be coplanar

This geometry causes strain due to both small bond angles and torsional strain

Conformation of Cyclobutane

Structure if constrained to plane actual structure

Cyclobutane adopts a “puckered” conformation in order to lower torsional strain Still have high bond angle strain

Conformation of Cyclopentane

The ring forms a preferred geometry to lower torsional strain

The conformation is called the “envelope” due to its similarity to a mailing envelope

Conformation of Cyclohexane

Cyclohexane has the least amount of ring strain

The reason is the ability of the ring to form a stable conformation

HH

H H

H H

H H

HH

HH

120˚ HH

H

HH

H

HH

HH

HH

111.4˚

Planar cyclohexane (120˚ <C-C-C,

All hydrogens eclipsed)

Chair cyclohexane (nearly tetrahedral <C-C-C,

no hydrogens eclipsed)

Names for Various Conformers of Cyclohexane

HH

H

HH

H

HH

HH

HH

H

H

H

HH

H

H

H

H

H

HH

H

H

HH

H

H

H

H

H

H

Remove hydrogens

Chair conformation Boat conformation Twist-boat conformation

Newman Projection for Chair Conformation

The chair conformation has a low torsional strain as seen in a Newman projection

Still have some gauche interactions, but energy is low for this conformation

Nearly perfect staggered alignment

Chair-Chair Interconversion with Cyclohexane

Key point – there are two distinct chair conformations for a cyclohexane that can interconvert

6-Membered Rings are Observed Frequently in Biological Molecules

Pole(axial)

equator

The 12 substituents in a chair (12 hydrogens for cyclohexane) occur in two distinct types of positions

HH

H H

H H

H H

HH

HH

In flat conformation, all hydrogens are identical

HH

H

HH

H

HH

HH

HH

In chair conformation, 2 sterically different positions occur

The Axial and Equatorial Positions have Different Spatial Requirements

There are two chair conformations, a substituent moves from equatorial to axial in a chair-chair interconversion

Y is equatorial Y is axial

Bigger Y substituent has more steric interactions in an axial position than equatorial

The chair conformation which has the Y group equatorial is therefore more stable

A substituent would prefer to be in an equatorial position

If there are two substituents they will compete for the equatorial position

An ethyl group is bigger than a methyl so therefore this compound would prefer the left conformation

If both substituents can be in the equatorial position than this conformer will be heavily favored

If there are more substituents, need to compare the cumulative sterics for all substituents to predict which chair is more stable

Don’t confuse Equatorial/Axial with Cis/Trans

A cis/trans ring junction refers to whether both substituents are on the same side or opposite sides of a ring