Unsaturated hydrocarbons

Unsaturated hydrocarbons Chapter 13

Unsaturated hydrocarbons • Hydrocarbons which contain at least one C-C multiple (double or triple) bond. • The multiple bond is a site for chemical reactions in these molecules. Parts of molecules where reactions can occur are called functional groups. Multiple bonds are examples of functional groups

Alkenes and cycloalkenes • Alkenes are unsaturated, acyclic hydrocarbons that possess at least one C-C double bond. • The generic formula for an alkene is CnH2n (note: same as for a cycloalkane).

Alkenes and cycloalkenes • Cycloalkenes are cyclic hydrocarbons that possess at least one C-C double bond. Cycloalkenes have a general formula of CnH2n-2

Alkenes and cycloalkenes • The geometry around the carbon atoms of the multiple bond is different than the tetrahedral geometry that is always found in carbon atoms of an alkane. • There is a trigonal planar arrangement of atoms surrounding the C-atoms of the double bond. see: VSEPR theory, Ch-5 Alkenes with two double bonds (dienes), three double bonds (trienes) are not uncommon. Cyclic alkenes usually do not involve more than one C-C double bond.

IUPAC nomenclature for alkenes and cycloalkenes • The rules for assigning an IUPAC name for alkenes are not that different from those for alkanes (substituent rules, chain numbering pretty much the same) • The difference here is that the longest continuous chain that has the double bond is the parent chain.

IUPAC nomenclature for alkenes and cycloalkenes • The parent chain is numbered to reflect the position of the double bond (the lower number of the two carbons in the bond).

IUPAC nomenclature for alkenes and cycloalkenes • For substituted alkenes, the number of the substituent is indicated as before, at the beginning of the name. For numbering, the parent chain is numbered in a way that gives the lowest numbering to the multiple bond(s). Substituent numbers are then assigned.

IUPAC nomenclature for alkenes and cycloalkenes • For dienes, the parent chain that involves both double bonds is numbered to show the first carbon in each double bond.

IUPAC nomenclature for alkenes and cycloalkenes • For cycloalkenes, the double bond in the ring is numbered only if more than one double bond exists (it is understood the C-1 is the first carbon of a double bond in a ring)

IUPAC nomenclature for alkenes and cycloalkenes • In certain cases, numbering is redundant (and not shown).

IUPAC nomenclature for alkenes and cycloalkenes • Later, in larger molecules that possess other groups of atoms (e.g. aromatics), alkene substituents may be present. The types we may encounter are named as follows:

Line-angle structural formulas for alkenes • Line-angle formulas for alkenes indicate double bonds with two lines. As before, each carbon must possess four bonds, so the number of H-atoms on each position will be able to be found by difference.

Constitutional isomerism in alkenes • For a given number of carbon atoms in a chain (> 4 C-atoms), there are more constitutional isomers for alkenes than for alkanes (because of the variability of the C-C double bond position) Rem: constitutional isomers differ in their atom-to-atom connectivity.

Constitutional isomerism in alkenes • Two types of constitutional isomers encountered are skeletal isomers and positional isomers. • Positional isomers are constitutional isomers that differ in the position of the multiple bond (or, in general, the functional group) • Skeletal isomers are constitutional isomers that differ in their C-chain (and thus H-atom) arrangements.

Cis-trans isomerism in alkenes • We’ve already looked at cycloalkanes and cis-, trans- isomers. In alkenes, this type of stereoisomerism is possible because a C-C double bond cannot rotate (like the C-C bonds in a cycloalkane ring). • For certain alkenes (which possess one H-atom on each carbon of the C-C double bond) there are two stereoisomers: cis- and trans- For cis-/trans- isomerism, there must be a H-atom and another group attached to each C-atom of the double bond cis: H-atoms on same side of C-C double bond trans: H-atoms on opposite sides of C-C double bond

Cis-trans isomerism in alkenes • For cis-, trans- isomerism, the alkene double bond cannot be located at the end of a carbon chain:

Cis-trans isomerism in alkenes • You can differentiate cis-/trans- isomers in line-angle structures:

Cis-trans isomerism in alkenes • For dienes, each bond is labeled as cis- or trans-, as required:

Cis-trans isomerism in alkenes • In some cases, you’ll encounter alkenes that have only one or no H-atoms bound to the C-atoms of the double bond. • For these cases, instead of cis- and trans- labels, (Z)- and (E)- labels (respectively) are used. CH3-CH2- substituent higher priority than CH3- substitutent This system works for more than just alkyl substituents, but we will stick to these cases for now. (E similar to trans- and Z similar to cis-) For both higher priority substituents on same side of double bond, (Z)- For higher priority substituents on opposite sides of double bond: (E)-

Physical properties of alkenes and cycloalkenes • Alkenes and cycloalkenes have solubilities similar to what was discussed for alkanes and cycloalkanes • Generally, alkenes have melting points that are lower than for corresponding alkanes

Chemical properties of alkenes and cycloalkenes • Like alkanes, combustion reactions can occur, producing H2O and CO2 • Other reactions of alkenes tend to involve the C-C double bond. These are addition-type reactions alkene alkane A-B “adds across” the C-C double bond. The double bond becomes transformed to a C-C single bond in the process

Chemical reactions of alkenes and cycloalkenes • Addition reactions can be symmetrical or unsymmetrical, depending on what is being added to the double bond. • In a symmetrical addition, the atoms (or groups) added to each carbon of the double bond are identical. Hydrogenation of an alkene Halogenation of an alkene

Chemical reactions of alkenes and cycloalkenes • Unsymmetrical addition reactions occur when different atoms (or groups) are added across a double bond. • Several examples of unsymmetrical addition reactions follow: • Hydrohalogenation of a double bond • Hydration of a double bond

Chemical reactions of alkenes and cycloalkenes • Hydrohalogenation: a hydrogen halide is added to a double bond; one C-atom becomes bound to the halogen and the other C-atom to a hydrogen: In general:

Chemical reactions of alkenes and cycloalkenes • Hydration reactions add a molecule of water to a double bond. The water molecule adds as HO-H: An alcohol (R-OH)

Chemical reactions of alkenes and cycloalkenes • In unsymmetrical addition reactions, if the alkene itself is not symmetrical, there will be more than one possible product. An unsymmetrical alkene is one for which the two C-atoms of the double bond are not equivalent.

Chemical reactions of alkenes and cycloalkenes • There will typically be one product in these cases that is favored (produced in greater yield). Markovnikov’s Rule states that when an unsymmetrical addition involves an unsymmetrical alkene, the H-atom of HX adds to the carbon of the double bond that has the most hydrogens. Major product Minor product

Chemical reactions of alkenes and cycloalkenes • For dienes and trienes, addition reactions (e.g. hydrogenation) will involve more than one of the double bonds, provided enough of the reactant (e.g. H2) is added:

Polymerization of alkenes • Alkenes (and alkynes) are able to undergo reactions that create long chains of atoms called polymers. In general, these reactions are called polymerization reactions. • Polymers are large molecules that are made up of repeated sequences of smaller units. The small molecules used to make the polymer are called monomers. • One of the bonds in the double bond is used to add the monomer structures into a growing polymer chain. The reaction is called addition polymerization. “n” expresses the average chain length Polyethylene

Polymerization of alkenes • Substituted alkenes can also undergo this type of reaction, yielding polymer chains that possess branches (substituents) • For dienes, polymerization yields polymers that contain double bonds: • In cases where two different monomers are involved, copolymer (containing both monomer units) are obtained.

Polymerization of alkenes • Polymers find many uses (plastics are polymers). • However, because they consist of alkane-type carbon chains, they are also unreactive. This means they don’t decompose readily in a landfill site. Monomer

Alkynes • Saturated hydrocarbons that possess at least one C-C triple bond are called alkynes. • For naming, the rules that were followed for alkenes are used, except that the name of the parent chain now ends in “yne”. General formula for alkyne: CnH2n-2

Alkynes • Because C-atoms only possess four covalent bonds, the C-atoms involved in the C-C triple bonds of alkynes possess local, linear molecular geometries. • This means that cis-, trans- isomers are not possible for alkynes (at the C-C triple bond). sp-hybridized carbons

Alkynes • However, constitutional isomers exist. Positional isomers C4H6 Skeletal isomers C5H8

Alkynes • The triple bond in an alkyne can undergo addition reactions similar to the double bond of an alkene: alkyne alkene alkane Two equivalent amounts of hydrogen added to an alkyne will make an alkane

Aromatic hydrocarbons • Aromatic hydrocarbons: a special class of cyclic, unsaturated hydrocarbons which do not readily undergo addition reactions. Benzene (C6H6) is an example of an aromatic hydrocarbon

Aromatic hydrocarbons • Benzene is a cyclic triene which possesses alternating C-C double and single bonds. • Because there are two ways the structure could be drawn, benzene is often represented with a circle-in-a-hexagon formula, showing the delocalization of the bonds. C6H6 = set of three delocalized bonds

Names for aromatic hydrocarbons • Benzene derivatives with one substituent • Certain cases have specific names

Names for aromatic hydrocarbons • In cases where a substituent name is not easily obtained, the benzene is called a “phenyl” substituent and the name is assigned using the alkane/alkene as the parent:

Names for aromatic hydrocarbons • Benzene derivatives with two substituents will have a bonding pattern that will fit one of the following schemes: “ortho” “meta” “para”

Names for aromatic hydrocarbons • This enables one of two possible naming schemes:

Names for aromatic hydrocarbons • When one of the special case compounds (e.g. toluene) is involved, the compound is named as a derivative of the special compound.

Names for aromatic hydrocarbons • In cases where disubstituted benzenes occur where substituents are not the same and where no special cases are involved, the substituent that has alphabetic priority also gets numbered on C-1.

Names for aromatic hydrocarbons • Disubstituted benzenes possessing two methyl substituents are a special case themselves. They are not called dimethyl benzenes or methyl toluenes, but instead are called xylenes: Fun fact! Three methyl groups on a benzene ring: named as a trimethylbenzene, not a methyl xylene.

Names for aromatic hydrocarbons • Three substituents: numbered to give the lowest possible numbering. Given a choice, alphabetic priority would dictate which substituent is on C-1.

Physical properties and sources of aromatic hydrocarbons • Similar to what we’ve seen for other hydrocarbons, aromatics are generally water-insoluble and have densities less than that of water. • Benzene is pretty good at dissolving other organic molecules (can serve as solvents for chemical reactions). • Industrially, aromatics are produced from saturated hydrocarbons:

Chemical reactions of aromatics • The double bonds of aromatic hydrocarbons are resistant to addition-type reactions. Instead, with the aid of catalysts, they can undergo substitution reactions: • Alkylation of benzene: + Chloroethane Ethylbenzene Benzene in general:

Chemical reactions of aromatics • Halogenation of benzene: in general:

Fused-ring aromatics • There are common cases of aromatic structures involving fused benzene rings:

Unsaturated hydrocarbons