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.
• Alkenes are unsaturated, acyclic hydrocarbons that possess at least one C-C double bond.
• Cycloalkenes are cyclic hydrocarbons that possess at least one C-C double bond (within the ring).
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
Propene 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 same) • The difference here is that the longest continuous chain that has the double bond is the parent chain.
correct parent chain not correct
Chain is numbered in the direction that gives the double bond(s) the lowest numbering. 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).
1-Butene 2-Butene 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.
1,4-Hexadiene 3,5-Dimethyl-1,3-hexadiene 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)
3-Ethylcyclohexene 1,3-cyclohexadiene 5-Ethyl-1,3-cyclohexadiene
In a cycloalkene, carbons 1 and 2 are automatically double bond carbons (count through the double bond when numbering the ring). IUPAC nomenclature for alkenes and cycloalkenes • In certain cases, numbering is redundant (and not shown).
Ethene Propene Methylpropene
where else could the double Only one carbon that a methyl group bond be, besides carbon 1? could be found on in propene 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.
1-Butene Propene 2-Methyl-2-pentene
2-Methyl-1,3-butadiene Non-IUPAC: isoprene 3,4-Dimethylcyclopentene 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 have same C- skeleton but 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. C5H10 1-Pentene 2-Pentene
skeletal isomers skeletal isomers
2-Methyl-2-butene Cis-trans isomerism in alkenes Stereoisomerism (again) • 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
H-atoms on same side H-atoms on opposite of C-C double bond sides of C-C 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:
This is true for any alkene that has two identical groups on one of the double bond carbons Cis-trans isomerism in alkenes
• You can differentiate cis-/trans- isomers in line-angle structures:
cis-2-Pentene Cis-trans isomerism in alkenes
• For dienes, each bond is labeled as cis- or trans-, as required:
trans-cis-2,4-Heptadiene cis-cis-2,4-Heptadiene Cis-trans isomerism in alkenes
• Cis-/trans- isomers are distinct molecules (i.e. they are different structures – not like conformers). • To transform one into the other, one of the bonds in the alkene double bond would need to be broken first – this requires energy (more energy than is available at room temperature) • If enough energy were available to do this, an isomerization reaction could occur (transforming one stereoisomer into the other)
Rem: breaking bonds costs energy Chemistry of vision retinal is a “polyene”
aldehyde group (will encounter these later)
Essentials of general, organic, and biochemistry. D. Guinn, R. Brewer, W.H. Freeman, NY, 2010. Cis-trans isomerism in alkenes
• Remember, C-atoms in double bonds (e.g. in alkenes) have trigonal planar molecular geometries.
Propene Cis-/trans- isomerism in alkenes
• Draw structures for the stereoisomers of 2- pentene
cis-2-pentene = 2-pentene
trans-2-pentene E-/Z- labels in stereochemistry
• 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. This system works for more than just CH3-CH2- substituent alkyl substituents, higher priority than (E similar to trans- and Z similar to cis-) but we will stick to CH3- substitutent these cases for now.
For both higher priority substituents on same side of double bond, (Z)- For higher priority substituents on opposite sides of double bond: (E)- E-/Z- labels in stereochemistry
• Priority is assigned on the basis of how many C-atoms are in the groups bound to the double bond C-atoms 2 C-atoms 1 C-atom (higher priority)
2 C-atoms 0 C-atoms (higher priority)
For both higher priority substituents on same side of double bond, (Z)- For higher priority substituents on opposite sides of double bond: (E)- When to use cis-trans vs. E-/Z-
• Look at the two C-atoms in the double bond. If both double bond carbons are each bound to one H-atom, use cis-/trans- • If the above statement isn’t true for the structure, use an E-/Z- label
trans-3-hexene (E)-3-methyl-3-hexene (or 3-methyl-(E)-3-hexene) Chemical reactions of alkenes and cycloalkenes • Like alkanes, combustion reactions can occur for
alkenes/cycloalkenes, producing H2O and CO2 • Another reaction of alkenes involves the C-C double bond, called an addition reaction
An example of a reaction that breaks a C-C bond
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 • Hydrohalogenation (e.g. bromination): 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:
Produces a HBr haloalkane
In general: HX
H+ catalyst HO-H
An alcohol (R-OH) This reaction more important than a hydrohalogenation reaction in the body 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 hydration addition reaction, H2O is added across the C=C double bond as H-OH, so it is considered to be unsymmetrical
H+ catalyst H2O
H+ catalyst H2O 3-Pentanol (one of two possible products) trans-2-Pentene Chemical reactions of alkenes and cycloalkenes • Unsymmetrical addition reactions occur when different atoms (or groups) are added across a double bond. Chemical reactions of alkenes and cycloalkenes • For unsymmetrical addition reactions, if the alkene itself is not symmetrical (around the C=C double bond), 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.
H-OH 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 tends to add to the carbon of the double bond that has the most hydrogens.
Minor product 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
• 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). Alkynes
• However, constitutional isomers exist.
Positional isomers C4H6
Skeletal isomers C5H8
1-Pentyne 3-Methyl-1-butyne Alkynes
• The triple bond in an alkyne can undergo addition reactions similar to the double bond of an alkene:
alkyne Ni (catalyst) alkene Ni (catalyst) alkane
Two equivalent amounts of hydrogen added to an alkyne will make an alkane
Notice: the end product is again an alkane. Addition reactions can’t break all bonds of a multiple bond. s- and p- bonds in unsaturated hydrocarbons • In a multiple bond, there is more than one bond type present. • Every single bond results from the “head-on” overlap of orbitals. The overlap of orbitals produces a bond.
This kind of bond is called a s (sigma) bond. All single bonds are s-bonds. Example: s- and p- bonds in unsaturated hydrocarbons • Multiple bonds have one s-bond, plus at least one pi-bond (p-bond)
p-bonds are created by the sideways overlap of parallel, atomic p-orbitals
Sideways overlap is not as strong as head-on overlap, so p-bonds are weaker than s-bonds. s- and p- bonds in unsaturated hydrocarbons • In a molecule that contains a double bond,
double bond = one s-bond + one p-bond s- and p- bonds in unsaturated hydrocarbons • For a molecule with a triple bond, there are two p-bonds and one s-bond:
triple bond = one s-bond + two p-bonds 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
Chlorobenzene tert-Butylbenzene Isopropylbenzene • Certain cases have specific names
• 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:
"phenyl" substituent Names for aromatic hydrocarbons
• Benzene derivatives with two substituents will have a bonding pattern that will fit one of the following schemes:
1,2-dibsubstituted 1,3-dibsubstituted “ortho” “meta” 1,4-dibsubstituted “para” Names for aromatic hydrocarbons
• This enables one of two possible naming schemes:
1,2-Dichlorobenzene 1,3-Dichlorobenzene (ortho-Dichlorobenzene) (meta-Dichlorobenzene)
ortho-Bromoiodobenzene meta-Bromopropylbenzene Names for aromatic hydrocarbons
• In cases where disubstituted benzenes occur where substituents are not the same, the substituent that has alphabetic priority also gets numbered on C-1.
1-Bromo-3-ethylbenzene 1-Bromo-2-chlorobenzene Names for aromatic hydrocarbons
• When one of the special case compounds (e.g. toluene) is involved, the compound can be named as a derivative of the special compound.
3-Bromotoluene 2-Ethyltoluene 2-Chlorostyrene or or or 1-bromo-3-methylbenzene 1-ethyl-2-methylbenzene 1-chloro-2-vinylbenzene 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.
1,2,4-Tribromobenzene 1-Bromo-3,5-dichlorobenzene Fused-ring aromatics
• There are common cases of aromatic structures involving fused benzene rings: