hapter 3: Structure and Nomenclature of rganic ompounds Focus on Alkanes rganic molecules are composed of one or more functional groups attached to one or more hydrocarbon groups (alkyl or groups) I. Functional groups eactive portion of molecule May be part of the hydrocarbon group ( =, = or benzene rings) or consist of atom groups bonded to the hydrocarbon moiety. lassification of organics is usually by functional group eactions of molecules with a given functional group follow same patterns II. ydrocarbon (alkyl or ) groups ydrocarbon portion of molecule ange in size from one carbon to twenty or more Different bonding arrangements are possible; isomerism is common 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 (2 + 2 + N X) IV. Structure of alkanes The general formula for alkanes (chain or branched) = n (2n+2) Saturated hydrocarbons arbon 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 onstitutional 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! -- onsider 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 onsider effect of surface area on intermolecular forces Branched vs. straight-chain structure Solubility properties are influenced by size and structure onsider how alkanes interact with solvents Like dissolves like principle - Water-soluble vs. fat soluble VI. hemical eactions of Alkanes: Alkanes are unreactive compared to other classes of organic compounds. Two reactions of alkanes: 1) ombustion Ex: 3 8 + 5 2 3 2 + 4 2) Free-radical halogenation hν Ex: 4 + Br 2 3 Br + Br
Effect of alkane size on melting and boiling points (Fig. 3.4)
ommon Functional Groups & Alkyl Groups in rganic ompounds Functional group ( where the action is ): eactive portion of molecule, may be a single atom or a group of atoms bonded in a specific arrangement Alkanes: n (2n+2) (no functional group!) Alkenes: n n Alkynes: n (2n-2) Alcohols: Thiols: S 3 3 3 ex: 3 ycloalkanes: also n n ex: Alkyl halides: X (X = F, l, Br or I) 3 Br Amines: -N 2 N, 3 N ex: 3 3 N 3 arboxylic acids: () Esters: ' Ketones: 3 Aldehydes (): Ethers: 3 3 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 / ydrogen bonding
Alkyl groups (""): The hydrocarbon portion of the molecule ( x y ), may vary in size and shape, appearing as chains, rings, or branches. Methyl: - 3 Ethyl: - 3 Propyl: - 3 7 2 types: n-propyl: isopropyl: 2 3 3 -------------------------------------------------------------------------------------------------------------------------- 3 Butyl: - 4 9 4 types: n-butyl: 3 Sec-butyl: isobutyl: tert-butyl: 3 3 3 3 3 3 3 ------------------------------------------------------------------------------------------------------------------------- yclohexyl: Neopentyl: 3 3 3 -------------------------------------------------------------------------------------------------------------------------- Phenyl: an aryl group Benzyl: Vinyl: Allyl:
ules for IUPA 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 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 AB 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 ). 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 AB order where possible. 8. While other functional groups have designated suffixes, alkyl halides are generally named as halo derivatives of a parent alkane.
onformations and energy in alkanes: Because alkanes have only single σ bonds, free rotation around the bonds is possible. I. In small alkanes, rotation about σ bond produces conformers Newman projections show the relative positions of groups on each carbon when sighting along a - 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: epulsion between nearby bonds; occurs when any atoms are eclipsed Steric strain: epulsion between atoms or groups trying to occupy same space; occurs with large groups that are eclipsed or gauche "Energy cost": ow much energy does it take to adopt a given conformation?
otation energy diagram for butane
Petroleum: Nature s alkanes Source: omposition: Decomposition of plant & animal matter Primarily straight-chain and branched alkanes Separation of components of crude oil in refineries is done by fractional distillation Distillation theory: A mixture is heated and allowed to vaporize & condense repeatedly in a distilling column efinery: Bubble-plate column Laboratory: olumn 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 30 o = natural gas ( 1 4 ) 30 200 o = gasoline ( 5 12 ) 175 300 o = kerosene, jet fuel ( 12 16 ) 200 350 o = diesel fuel ( 8 18 ) 275 400 o = heating oil ( 15 20 ) > 350 o = lube oil, waxes ( 18 and up) Alkanes are primarily used as fuel because their most common reaction is combustion: 4 + 2 2 2 + = - 213 kcal/mol 8 18 + 25 / 2 2 8 2 + 9 = - 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 atalytic cracking of larger alkanes produces smaller ones 10 4 heat, Pt or 2 5 12 Silica/alumina
Some basic hydrocarbon skeletons As size increases, more isomers become possible
Naming branched alkanes: find the longest chain and number carbons accordingly
onvert each one to a line-bond structure, then name it