Chapter 3: Structure and Nomenclature of Organic Compounds Focus on Alkanes

Organic molecules are composed of one or more functional groups attached to one or more hydrocarbon groups (alkyl or “R” groups)

I. Functional groups

• Reactive portion of molecule • May be part of the hydrocarbon group (C = C, C = C or benzene rings) or

consist of atom groups bonded to the hydrocarbon moiety. • Classification of organics is usually by functional group • Reactions of molecules with a given functional group follow same patterns

II. Hydrocarbon (alkyl or “R”) groups

• Hydrocarbon portion of molecule • Range in size from one carbon to twenty or more • Different bonding arrangements are possible; isomerism is common • R influences physical properties • May participate in some chemical reactions, but often left unchanged • Learn structures & names of common small alkyl groups (see chart)

III. Determination of possible structures from a formula:

Useful tool: the degrees of unsaturation formula: Number of pi bonds & rings can be calculated from molecular formula (U) = ½ (2C + 2 – H + N – X)

IV. Structure of alkanes

The general formula for alkanes (chain or branched) = CnH(2n+2) • Saturated hydrocarbons • Carbon arrangement may include straight chains, branched chains or rings

as well as combinations of these structures • Many possible bonding arrangements arising from a given formula leads to

Constitutional isomerism Isomers: one or more different structures having the same formula

V. Effect of Structure on Physical Properties of Alkanes

Many physical properties depend on intermolecular forces between molecules Boiling and melting points

• Alkane size and molecular weight affects melting and boiling points:

Intermolecular forces in alkanes are London dispersion forces only! -- Consider the effect of molecule size on London forces

-- Petroleum refining takes advantage of how boiling point changes with molecule size (lab/industrial method: fractional distillation)

• Branching affects boiling points Consider effect of surface area on intermolecular forces Branched vs. straight-chain structure

Solubility properties are influenced by size and structure

• Consider how alkanes interact with solvents

• “Like dissolves like” principle - Water-soluble vs. “fat” soluble

VI. Chemical Reactions of Alkanes:

Alkanes are unreactive compared to other classes of organic compounds. Two reactions of alkanes: 1) Combustion Ex: C3H8 + 5 O2 3 CO2 + 4 H2O

2) Free-radical halogenation hν Ex: CH4 + Br2 CH3Br + HBr

Effect of alkane size on melting and boiling points (Fig. 3.4)

Common Functional Groups & Alkyl Groups in Organic Compounds Functional group (“where the action is”): Reactive portion of molecule, may be a single atom or a group of atoms bonded in a specific arrangement Alkanes: CnH(2n+2) (no functional group!) ex: Alkenes: CnH2n Cycloalkanes: also CnH2n ex: Alkynes: CnH(2n-2) Alcohols: R – OH Alkyl halides: R – X (X = F, Cl, Br or I) Thiols: R – SH Amines: R-NH2 R2NH, R3N ex: Carboxylic acids: Esters:

(RCOOH) Ketones:

Aldehydes (RCHO): Ethers: Epoxides: Section 2.13: Non-covalent interactions - intermolecular forces between organic molecules depends strongly on what kind of functional group is present (if any) Nonpolar molecules: Dispersion (London) forces only Polar molecules: Dipole-dipole forces / Hydrogen bonding

R

HC

CH

R

CH2

H2C

CH2

CH3H3C

R C CH

OCH3H3C

R

HC

HC

RO

RC

O

OH RC

O

OR'

CH3C

CH3

OH

CH3 Br

N

CH3

CH3H3C

RC

O

CH3 RC

O

H

H2C

H2CCH2

CH2

H2C

Alkyl groups ("R"): The hydrocarbon portion of the molecule (CxHy), may vary in size and shape, appearing as chains, rings, or branches.

Methyl: -CH3 Ethyl: -CH2CH3 Propyl: -C3H7 2 types: n-propyl: isopropyl: -------------------------------------------------------------------------------------------------------------------------- Butyl: -C4H9 4 types: n-butyl: Sec-butyl: isobutyl: tert-butyl: ------------------------------------------------------------------------------------------------------------------------- Cyclohexyl: Neopentyl: -------------------------------------------------------------------------------------------------------------------------- Phenyl: Benzyl: an “aryl” group Vinyl: Allyl:

CH2

H2C

CH2

CH3

H3C

HC

CH2

CH3

H2C

CHCH3

CH3

CHCH3

CH3

H2C

CH2

CH3

H3CC

CH3

CH3

H2C

HC

CH2CH2

HC CH2

H2C

CCH3

CH3

CH3

Rules for IUPAC naming of commonly encountered alkanes & alkyl halides 1. Find the longest continuous chain of carbons in the structure. This is the parent hydrocarbon chain. The parent chain’s name is based on the number of carbons: root prefix + "ane"

Ex: For 5 C chain, root = “pent”, parent name = “pentane” 2. Number the chain starting at the end closest to the substituents. The numbering should lead to the smallest possible numbers for all substituents. 3. Name the substituents and assign numbers to each. They should if possible be numbered such that the first group in ABC order gets the lowest number. 4. Substituents which appear more than once are grouped together with a prefix:

“di-” = 2 “tri-” = 3 “tetra-” = 4 “penta-” = 5 “hexa” = 6 5. The compound name is arranged with the number and name of each substituent first, separated by hyphens. After the last substituent name, the root name is added. Prefix-Parent-Suffix 6. For cycloalkanes, the parent name of the compound is derived from the ring (unless it is attached to a chain with a greater number of C). Parent = “cyclo” + “root prefix” + “ane” 7. Substituents on the ring should be numbered such that they receive the lowest possible numbers. They should be numbered by ABC order where possible. 8. While other functional groups have designated suffixes, alkyl halides are generally named as halo derivatives of a parent alkane.

Conformations and energy in alkanes: Because alkanes have only single σ bonds, free rotation around the bonds is possible. I. In small alkanes, rotation about C – C σ bond produces conformers

Newman projections show the relative positions of groups on each carbon when sighting along a C-C bond Potential energy vs. rotation plots show the energy differences between rotational conformations of alkanes. Larger molecules = more conformations! Types of conformations and their energies: For ethane, only two exist: staggered < eclipsed For larger alkanes such as butane: staggered < staggered < eclipsed anti gauche Types of strain that may exist in regular alkanes or alkyl groups: Torsional strain: Repulsion between nearby bonds; occurs when any atoms are eclipsed Steric strain: Repulsion between atoms or groups trying to occupy same space; occurs with large groups that are eclipsed or gauche "Energy cost": How much energy does it take to adopt a given conformation?

Rotation energy diagram for butane

Petroleum: Nature’s alkanes Source: Decomposition of plant & animal matter Composition: Primarily straight-chain and branched alkanes Separation of components of crude oil in refineries is done by fractional distillation

Alkanes are primarily used as fuel because their most common reaction is combustion: CH4 + 2 O2 CO2 + H2O H = - 213 kcal/mol

C8H18 + 25/2 O2 8 CO2 + 9 H2O H = - 1308 kcal/mol Engine “knocking” occurs with certain hydrocarbon fuels, especially straight-chains Branched alkanes like 2,2,4-trimethylpentane have a higher “octane rating” Catalytic cracking of larger alkanes produces smaller ones

C10H24 heat, Pt or 2 C5H12 Silica/alumina

Distillation theory: A mixture is heated and allowed to vaporize & condense repeatedly in a distilling column Refinery: Bubble-plate column Laboratory: Column packed with glass beads or “steel sponge” The lower the boiling point, the easier it is for compounds to stay in the vapor phase, so they rise more quickly up the column and are collected in fractions: Below 30oC = “natural gas” (C1 – C4) 30 – 200oC = gasoline (C5 – C12) 175 – 300oC = kerosene, jet fuel (C12 – C16) 200 – 350 oC = diesel fuel (C8 – C18) 275 – 400oC = heating oil (C15 – C20) > 350oC = lube oil, waxes (C18 and up)

Some basic hydrocarbon skeletons

As size increases, more isomers become possible

Naming branched alkanes:find the longestchain and number carbons accordingly

Convert each one to a line-bond structure, then name it