Hydrocarbons Flashcards

Hydrocarbons

Carbon Bonding

Carbon has four valence electrons and always forms four covalent bonds.
This allows carbon to form very long chains and rings which then form proteins, carbohydrates, nucleic acids, etc.

Types of Hydrocarbons

Alkanes - Hydrocarbons with only single C-C covalent bonds.
- Also known as saturated hydrocarbons because they are saturated with hydrogens.
Alkenes - Hydrocarbons that contain at least one double C=C bond.
Alkynes - Hydrocarbons that contain at least one triple C≡C bond.

Aliphatic vs. Aromatic Hydrocarbons

Aliphatic Hydrocarbons - Continuous and branched chain hydrocarbons or nonaromatic rings.
- Can be alkanes, alkenes, or alkynes.
  - Suffixes are –ane, -ene, or –yne.
- Nonaromatic cyclic hydrocarbons:
  - Prefix of cyclo.
  - Ex: Cyclopentane, cyclohexane, cyclopentene, etc.
Aromatic Hydrocarbons - Cyclic, coplanar (flat) structures that have alternating double and single bonds and follow Hückel’s Rule.
- Benzene $(C6H6)$

Naming Hydrocarbons (IUPAC System)

Continuous and Branched Chain Alkanes, Alkenes, or Alkynes

Contain any number of carbon atoms connected in a straight or branched chain.
Use prefixes to denote the number of carbons in a chain:
- Meth, eth, prop, but, pent, hex, hept, oct, non, dec, undec, dodec, tridec, tetradec, pentadec, etc.

Substituent Group

An atom or group of atoms that replaces a hydrogen atom on the chain or ring.

Common Substituent Groups

Halogens
- Fluoro (F-), chloro (Cl-), bromo (Br-)
Alkyl (Carbon) Groups
- Methyl (CH3-)
- Ethyl (CH3-CH2-)
- Propyl (CH3-CH2-CH2-)
- Butyl (CH3-CH2-CH2-CH2-), etc.
- Phenyl (C6H5-)
Nitrogen Groups
- -NO2 = nitro group

Naming Conventions

Name the Chain:
- Find the longest continuous chain of carbons in the compound.
- Number the carbons in the main chain in sequence. If there is a single multiple bond present, carbon #1 will be on the end of the chain nearest the multiple bond. If there are two or more multiple bonds present, carbon #1 will be on the end of the chain that results in the lowest numbered carbons having multiple bonds.
- When there is a multiple bond(s) present in the main carbon chain, substituents DO NOT play a role in how the carbons are numbered!
- Number the multiple bonds by their position on the chain.
 - Ex. 2-butene $CH3-CH=CH-CH3$
- If double bonds or triple bonds are present, the suffix is -ene or -yne.
- More than one double bond: diene, triene, etc.
 - Ex. 1,3-hexadiene $CH2=CH-CH=CH-CH2-CH_3$ *NOT 3,5-hexadiene
Name the Substituent groups:
- Find the longest continuous chain of carbons in the compound.
- Number the carbons in the main chain in sequence. If an alkane, then carbon #1 will be on the end of the chain that results in the lowest numbered carbons having substituents attached to them. If two or more sidechains are in equivalent positions, assign the lowest number to the substituent whose name comes first alphabetically.
- Add numbers to the substituent or alkyl groups to indicate their position on the main chain.
  - 2-methyl, 3-ethyl, 1-fluoro, etc.
- Add simple prefixes for multiple substituent groups of the same type (i.e. 1,2-dichloro, 2,3,4-trimethyl, etc.) - each group gets a number.
- Arrange substituent groups in alphabetical order of substituent group name.

Examples

$CH3-CH-CH2-CH-CH2-CH2-CH3$ $\ \ \ \ \ | \ \ \ \ |$ $\ \ \ \ Cl \ \ \ \ CH3$
- 2-chloro-4-methylheptane
$CH3-CH=C-CH2-CH2-CH-CH2-CH2-CH3$ $\ \ \ \ | \ \ \ \ \ \ \ |$ $\ \ \ \ CH3 \ \ \ \ \ CH2-CH_3$
- 6-ethyl-3-methyl-2-nonene OR 6-ethyl-3-methylnon-2-ene

Structural Formulas

All carbons are attached to one another, and single covalent bonds to the hydrogen atoms are understood.

Examples

$CH3-CH2-CH2-CH2-CH2-CH3$ - Hexane
$CH3-CH=C=CH-CH3$ - 2,3-pentadiene OR penta-2,3-diene
$H-C≡C-H$ - Ethyne
$CH3-CH-CH2-CH-CH2-CH-CH2-CH3$ $\ \ \ | \ \ \ \ | \ \ \ \ |$ $\ \ CH3 \ \ \ CH3 \ \ \ CH3$ - 2,4,6-trimethyloctane

Cyclic Hydrocarbons

5 and 6 carbon rings are most common.
Cycloalkenes - cyclic hydrocarbons with double bonds (Ex. Cyclopentene)
Aromatic Hydrocarbons (Arenes)
- Benzene $(C6H6)$ is the most common arene. This is the IUPAC name for this compound.
Count the carbons in the ring, use IUPAC hydrocarbon names and add the prefix cyclo.
- Cyclohexane, cyclopentane, etc.
- If the ring is benzene or a cycloalkane, #1 carbon is the one that results in the lowest numbered carbons having substituents attached to them. You can go clockwise or counterclockwise to get the lowest total numbers for the substituent groups. If there is a tie, use the first alphabetical substituent.
- If the ring is a cycloalkene, number the carbons in the ring either clockwise or counterclockwise such that the lowest numbered carbon(s) contain multiple bond(s). Substituents do not influence the carbon numbering.

Cyclic Hudrocarbon Examples

4-bromo-1-chloro-2-methylcyclopentane
1-chloro-2-fluoro-4-methylcyclohexene
4-chloro-2-ethyl-1-methylbenzene

Shorthand for Denoting Cyclic Hydrocarbons

Each "joint" is a carbon, and all hydrogens are "understood," but not written.

Examples

Cyclohexane
Cyclopentene
1,3-cyclopentadiene / Cyclopenta-1,3-diene
Benzene

Isomers

Isomers are compounds with the same overall molecular formula but different properties.

Structural/Constitutional Isomers

Compounds with the same molecular formula but different molecular structures.
- Butane and 2-methylpropane
 - $CH3-CH2-CH2-CH3 \text{ VS } CH3-CH-CH3$
 $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |$
 $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_3$

Stereoisomers

Compounds that differ only in the geometry of their substituent groups.

Geometric/Cis-Trans Isomers

Can occur in either cyclic compounds or compounds that contain double bonds.
- Cis configuration – substituent groups on the same side of the double bond.
- Trans configuration – substituent groups are on the opposite sides of the double bond.
  - cis-2-butene VS trans-2-butene

Geometric/Cis-Trans Isomers Examples

Draw cis-1,2-dichloroethene and trans-1,2-dichloroethene
- cis-1,2-dichloroethene
  $H \ \ \ \ Cl$
  $\ \backslash C=C /$
  $\ \ \ / \ \ \ \ \ /$
  $\ \ \ Cl \ \ \ \ H$
- trans-1,2-dichloroethene
  $H \ \ \ \ Cl$
  $\ \backslash C=C /$
  $\ \ \ / \ \ \ \ \ /$
  $\ \ \ H \ \ \ \ Cl$
*Biochemical Example:
$11-cis \ retinal \ \ \xrightarrow{light} \ \ \ all-trans \ retinal$

Enantiomers

Occurs whenever a central atom (like carbon) has four DIFFERENT atoms or groups attached
- Chirality – It can be distinguished from its mirror image CHFClBr

Chiral Objects

Cannot be superimposed

Achiral Objects

Can be superimposed

Enantiomers Example

Thalidomide with $R$ and $S$ enantiomers is shown. The images show the chemical structure with the chiral carbon and which groups are attached with wedged and dashed bonds to show stereochemistry.