Unsaturated hydrocarbons. Chapter 13

Transcription

1 Unsaturated hydrocarbons Chapter 13

2 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

3 Alkenes and cycloalkenes Alkenes are unsaturated, acyclic hydrocarbons that possess at least one C-C double bond. The generic formula for an alkene is C n H 2n (note: same as for a cycloalkane). Ethene Non IUPAC: "ethylene" Propene Non-IUPAC: "propylene"

4 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 C n H 2n-2

5 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 o o Propene

6 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.

7 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

8 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.

9 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

10 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).

11 IUPAC nomenclature for alkenes and cycloalkenes In certain cases, numbering is redundant (and not shown). Ethene Propene Methylpropene where else could the double bond be, besides carbon 1? Only one carbon that a methyl group could be found on in propene

12 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

13 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 atomto-atom connectivity.

14 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. 1-Pentene C5H10 2-Pentene positional isomers skeletal isomers skeletal isomers 2-Methyl-2-butene

15 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 of C-C double bond H-atoms on opposite 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

16 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

17 Cis-trans isomerism in alkenes You can differentiate cis-/trans- isomers in line-angle structures: = = trans-2-pentene = = cis-2-pentene

18 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

19 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

20 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.

21 Cis-trans isomerism in alkenes Remember, C-atoms in double bonds (e.g. in alkenes) have trigonal planar molecular geometries. 120 o o Propene

22 Cis-/trans- isomerism in alkenes Draw structures for the stereoisomers of 2- pentene 2-pentene cis-2-pentene = trans-2-pentene

23 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. CH 3 -CH 2 - substituent higher priority than CH 3 - substitutent (E similar to trans- and Z similar to cis-) This system works for more than just alkyl substituents, but we will stick to 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)-

24 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 1 C-atom 2 C-atoms (higher priority) 2 C-atoms (higher priority) 0 C-atoms For both higher priority substituents on same side of double bond, (Z)- For higher priority substituents on opposite sides of double bond: (E)-

25 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)

26 Chemical reactions of alkenes and cycloalkenes Like alkanes, combustion reactions can occur for alkenes/cycloalkenes, producing H 2 O and CO 2 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

27 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: HBr Produces a haloalkane In general: HX where HX is HF, HCl, HBr, HI

28 Chemical reactions of alkenes and cycloalkenes Hydration reactions add a molecule of water to a double bond. The water molecule adds as HO-H: HO-H H + catalyst This reaction more important than a hydrohalogenation reaction in the body An alcohol (R-OH)

29 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, H 2 O is added across the C=C double bond as H-OH, so it is considered to be unsymmetrical H2O H + catalyst Ethene Ethanol H2O H + catalyst 3-Pentanol (one of two possible products) trans-2-pentene

30 Chemical reactions of alkenes and cycloalkenes Unsymmetrical addition reactions occur when different atoms (or groups) are added across a double bond.

31 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

32 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. Major product H-OH Minor product

33 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: C n H 2n-2 Ethyne (Acetylene) Propyne (Methylacetylene) 6,6-Dimethyl-3-heptyne

34 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).

35 Alkynes However, constitutional isomers exist. Positional isomers C 4 H 6 2-Butyne 1-Butyne Skeletal isomers C 5 H 8 1-Pentyne 3-Methyl-1-butyne

36 Alkynes The triple bond in an alkyne can undergo addition reactions similar to the double bond of an alkene: H2 H2 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.

37 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. Example: This kind of bond is called a s (sigma) bond. All single bonds are s-bonds.

38 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.

39 s- and p- bonds in unsaturated hydrocarbons In a molecule that contains a double bond, like H 2 CO: s-bonds s-bond double bond = one s-bond + one p-bond

40 s- and p- bonds in unsaturated hydrocarbons For a molecule with a triple bond, there are two p-bonds and one s-bond: s-bond s-bond triple bond = one s-bond + two p-bonds

41 Aromatic hydrocarbons Aromatic hydrocarbons: a special class of cyclic, unsaturated hydrocarbons which do not readily undergo addition reactions. Benzene (C 6 H 6 ) is an example of an aromatic hydrocarbon

42 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. = C 6 H 6 = set of three delocalized bonds

43 Names for aromatic hydrocarbons Benzene derivatives with one substituent Chlorobenzene tert-butylbenzene Isopropylbenzene Certain cases have specific names Toluene or Methylbenzene Styrene or Vinylbenzene "vinyl" substitutent

44 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: 2-Phenyl-2-butene 3-Phenylhexane "phenyl" substituent

45 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 ortho 1,3-dibsubstituted meta 1,4-dibsubstituted para

46 Names for aromatic hydrocarbons This enables one of two possible naming schemes: 1,2-Dichlorobenzene (ortho-dichlorobenzene) 1,3-Dichlorobenzene (meta-dichlorobenzene) 1,4-Dichlorobenzene (para-dichlorobenzene) ortho-bromoiodobenzene meta-bromopropylbenzene

47 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

48 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 or 1-bromo-3-methylbenzene 2-Ethyltoluene or 1-ethyl-2-methylbenzene 2-Chlorostyrene or 1-chloro-2-vinylbenzene

49 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

50 Fused-ring aromatics There are common cases of aromatic structures involving fused benzene rings: Napthalene Anthracene Phenanthrene