Hydrocarbon Nomenclature Organic Chemistry Lesson # 1

Introductory Terms Organic – compounds that contain carbon bonded to each other, hydrogen, and other specific non-metals like oxygen and nitrogen. Exceptions include: carbonates (CO32-), cyanides (CN-), carbides (C22-), and oxides of carbon (CO2, CO). The definition used to be any carbon compound, or any compounds that support life. Inorganic – any compound that does not contain carbon and the exceptions listed above. Organic Chemistry – the study of carbon-containing compounds and their properties.

Introductory Terms Hydrocarbon – a compound containing only carbon and hydrogen atoms. They can be saturated, where all the bonds between carbon atoms are single bonds, or unsaturated, where there is at least one double or trip bond between carbon atoms. Functional Group – a group of atoms within a molecule that determine the properties of the molecule, such as solubility or reactivity. This could be as simple as a single atom or a type of bond, or as complex as a group of atoms.

Why Carbon? Carbon allows for the formation of a large variety of compounds. Carbon atoms have the ability to form 4 bonds. Carbon atoms may bind to form chains, rings, spheres, sheets, and tubes. Carbon atoms have the ability to form single, double, and triple bonds.

Alkanes The simplest type of hydrocarbon, and is saturated with all single bonds. Are in a chain structure (straight line). Methane is the simplest with only one carbon, which means it is surrounded by 4 hydrogens. The bond angles between the hydrogens are all 109.5° and it forms a tetrahedral shape. You know methane as natural gas. Other common alkanes are propane and butane, which are used to heat homes and fuel tools. Carbon-carbon single covalent bonds are held tightly together – the bond is difficult to break. This is because there are no unpaired electrons to participate in a reaction, therefore we say the bond is unreactive. The general formula is CnH2n+2

First 5 Alkanes Methane -162°C Ethane -89 °C Propane -42 °C Butane 0 °C Pentane 36 °C

Alkanes Homologous Series – in a group of compounds with the same general formula, as the molar mass increases, a physical property increases. In this case, boiling point. This is also true of melting point. This has to do with intermolecular forces. The C-H bond is non-polar as there is only a very small difference in electronegativity. Weak London dispersion forces (also known as Van der Waals forces) hold molecules together, and the more atoms, the more forces exist. Chemists take advantage of the homologous series when trying to separate a mixture of hydrocarbons in a process called fractional distillation.

Cyclic Alkanes Hydrocarbons that exist in an enclosed ring structure instead of a chain. You need at least three carbons to form a cyclic structure. Rings of 3 or 4 tend to be reactive as they have small bond angles, which are unstable. General Formula: CnH2n Cyclopropane Cyclohexane

Structural Isomers Structural isomers are compounds that have the same chemical formula but different structural formula. We can re-arrange the atoms in some straight-chained hydrocarbons so that they exist as “branches”. Any attachment to a chain that replaces a hydrogen atom is known as a substituent group. When the replacement is carbon, it is specially known as an alkyl group. Straight-chain and cyclic alkanes are not isomers of one another as they have different general formulas.

Structural Isomers of… 1. Butane 2. Pentane

Drawing Practice a) 3-methylpentane b) 2,3-dimethylhexane c) 4-ethyloctane

Drawing Practice d) 1,4-dimethylcyclohexane e) chlorobutane f) 2,2-dibromopropane

Naming Practice a) b) c)

Naming Practice d) g) e) f)

Alkenes & Alkynes Two unsaturated groups of hydrocarbons where at least one double or triple bond exists in the parent chain. Alkenes contain double bonds, and alkynes contain triple bonds. When double or triple bonds form between carbons they become easier to break (triple bonds are weaker than double, which are weaker than single). These bonds are said to be reactive, as the double or triple can act as a “reaction centre” (more on this later). The most simple alkene, ethene, is used in agriculture to help ripen fruit faster, and ethyne is used in welding (its old name is acetylene). Alkenes and alkynes are also non-polar.

Alkenes & Alkynes General Formula for Alkenes: CnH2n General Formula for Alkynes: CnH2n-2 Ethene Propyne Pent-2-ene

Naming & Drawing Alkenes/Alkynes Determine the parent chain and the location of the double or trip bond. When numbering the parent chain, begin with the end closest to the multiple bond (instead of branches). Alkenes end in “ene”, alkynes end in “yne”, and the numbering is between the prefix and suffix. If there is more than one multiple bond, the prefix di-, tri-, etc. are used.

Drawing Practice a) but-1-ene b) hex-3-yne c) 2,4-dimethylpent-2-ene

Drawing Practice d) octa-2,4,6-triene g) 4-methylcyclopent-1-ene e) hepta-3,4-diene f) 5-chlorohept-2-yne

Naming Practice a) b) c)

Naming Practice d) e) f)

Stereoisomers We know already that structural isomers have the same chemical formula but different structures. Stereoisomers have the same chemical formula formula and structural backbone, but with a different arrangement of atoms in space. An example common in double bonds is cis-trans isomerism. A cis isomer is when two substituent groups are located on the same side of a double bond, and a trans isomer is when they are on opposite sides. This gives rise to different physical properties.

Stereoisomers cis-pent-2-ene trans-pent-2-ene cis-1,2-dichloroethene trans-1,2-dichloroethene

Aromatic Hydrocarbons Alkanes, alkenes, and alkynes are all called aliphatic structures. They are structures based on branches or rings of carbon structures. Aromatic hydrocarbons are unsaturated cyclic hydrocarbons with a pattern of bonding that makes it chemically stable. The simplest aromatic hydrocarbon is benzene, with is a flat 6-carbon ring with alternating single and double bonds. Although there are two types of bonds, chemical analysis has show that the bond lengths are all equal. This is because the double bonds alternate positions constantly (the electrons are delocalized), making each carbon-carbon a 1.5 bond at any given moment in time. This is known as a resonance structure (more on this later).

Aromatic Hydrocarbons Aromatic hydrocarbons can be liquids at room temperature, or crystalline solids. They are symmetrical, so they are non-polar, which means they tend to be insoluble in water. Benzene is less reactive than alkenes. Benzene was once widely used as a solvent, but it is now known to be carcinogenic. General Formula: CnHn Benzene:

Naming Aromatic Hydrocarbons A benzene ring can be the main chain, or it can exist as a branch. When a benzene is a substituent group (branch) on a hydrocarbon chain, it is called a phenyl group. When benzene is the parent chain, number carbon 1 as the branch in lowest alphabetical order.

Naming Practice a) b) c)

Drawing Practice a) 4,6-diphenyloct-2-ene b) 1-ethyl-2,4-dimethylbenzene c) 2phenylbut-2-ene

Summary Alkanes Alkenes Alkynes Aromatics Saturated Unsaturated Single Bonds Double Bonds Triple Bonds 1.5 Bonds End in –ane End in –ene End in –yne End in –benzene CnH2n+2 CnH2n CnH2n-2 CnHn Non-polar Least reactive More reactive Most reactive Less reactive *Rings structure use cyclo- *Consider cis vs. trans *As substituent use phenyl-