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: + Benzene Chloroethane Ethylbenzene 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:

Hybrid orbitals and bonding in organic compounds We have seen that carbon adopts several bonding formats: Carbon’s electron configuration is 1s22s22p2. Its valence orbitals are the 2s and 2p orbitals.

Hybrid orbitals and bonding in organic compounds Bonds are created between atoms when electrons are shared. In order for electron-sharing to occur, the orbitals containing these electrons (on each atom) must overlap. This model can’t explain the observed bond angles in molecules like the ones shown below (shapes and orientations of the valence orbitals are incorrect):

Hybrid orbitals and bonding in organic compounds To explain bonding in these cases, a new model is used (called “Valence Bond Theory”) in which atomic orbitals (2s, 2p, etc.) are mixed to produce hybrid orbitals, which have directions that depend on the number of atomic orbitals mixed. sp-hybrid orbitals sp3-hybrid orbitals sp2-hybrid orbitals

Hybrid orbitals and bonding in aromatic compounds For a tetrahedral carbon (e.g. an alkane carbon), there are four other atoms bound to the C-atom. The molecular geometry around carbon is tetrahedral. A sp3-hybrid orbital set is used for explaining the tetrahedral arrangement. Hybrid orbitals point in the same directions as electron groups in VSEPR theory

Hybrid orbitals and bonding in aromatic compounds For a trigonal planar carbon, three atomic orbitals are combined to make three, sp2-hybrid orbitals.

Hybrid orbitals and bonding in organic compounds Linear carbons: involved in a triple bonds. Two atomic orbitals are combined to make a new hybrid orbital set (two sp-hybrid orbitals)

Hybrid orbitals and bonding in organic compounds In C-C single bonds, the bond is created by the overlap of orbitals in a head-on fashion. The situation is similar to what occurs when two H-atoms bond (or H and Cl-atoms): This is called a sigma bond (s-bond) (strong) What about multiple bonds? How do they form?

Hybrid orbitals and bonding in organic compounds Multiple bonds involve one s-bond, plus at least one pi-bond (p-bond) (one p-bond in a double bond or two p-bonds in a triple bond) p-bonds are created by the sideways overlap of parallel, atomic p-orbitals

Hybrid orbitals and bonding in organic compounds In a molecule that contains a double bond, like H2CO: double bond = s-bond + p-bond s-bonds s-bond sp2-hybrid orbitals are used to create the trigonal planar molecular geometry and the unused p-orbital is used to make the p-bond

Hybrid orbitals and bonding in organic compounds For a molecule with a triple bond, there are two, unused p-orbitals that can be used to make p-bonds: triple bond = s-bond + two p-bonds s-bond s-bond sp-hybrid orbitals create the linear molecular geometry around the C-atoms and two, unused p-orbitals on each C-atom can be used to make two p-bonds with the second carbon