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). Ethene Propene Non IUPAC: "ethylene" Non-IUPAC: "propylene" Alkenes and cycloalkenes • Cycloalkenes are cyclic hydrocarbons that possess at least one C-C double bond (within the ring). Cyclopentene 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 120o 109.5o 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. 2-Methyl-2-butene 3-Methyl-1-butene 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 positional isomers 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: 1-pentene 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: = = trans-2-Pentene = = cis-2-Pentene Cis-trans isomerism in alkenes • For dienes, each bond is labeled as cis- or trans-, as required: trans-trans-2,4-Heptadiene cis-trans-2,4-Heptadiene 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. 120o 109.5o 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. (E)-3-Methyl-3-hexene (Z)-3-Methyl-3-hexene 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 where HX is HF, HCl, HBr, HI Chemical reactions of alkenes and cycloalkenes • Hydration reactions add a molecule of water to a double bond. The water molecule adds as HO-H: 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 Ethene Ethanol 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.