hapter 13 Alkenes and Alkynes Types of Bonds Alkanes and haloalkanes consist of atoms held together by bonds. These can also be called sigma bonds (σ bonds) which means that the orbitals of each atom overlap in the space where we draw the bond: The first bond formed between two atoms will always be a sigma bond; however, two atoms cannot have more than one sigma bond holding them together. So, how do we make double and triple bonds? We use a second kind of bond called a pi bond (π bond) which means that the orbitals of each atom overlap in the space above and below the bond:
A double bond contains A triple bond contains Pi bonds are weaker than sigma bonds *but* the combination of a pi and a sigma bond is stronger than a sigma bond alone. Because the pi bonding occurs above and below the sigma bond, double and triple bonds are also rigid and cannot be rotated about. Saturation Like alkanes, alkenes and alkynes (or the alkene/alkyne part of a larger molecule) contain two elements, and. Alkenes contain at least one carbon-carbon double bond. Alkynes contain at least one carbon-carbon triple bond. Alkanes are often called saturated hydrocarbons because they contain as many atoms as possible for the number of atoms. They are saturated with. Alkenes and alkynes, on the other hand, have fewer atoms for the same number of atoms so they are often called unsaturated hydrocarbons since they are not saturated with.
Naming Alkenes and Alkynes Alkenes and alkynes are named according to IUPA rules (see summary in hapter 12 notes). Note that: Alkenes and alkynes are more important functional groups that haloalkanes. The main chain must contain all double/triple bonds and should be numbered from the end closest to the first double/triple bond. The name now ends in ene (for alkenes) or yne (for alkynes). Prefixes to indicate multiple double/triple bonds go immediately before the ene or yne e.g. pentadiene decatriyne Numbers to indicate the location of double/triple bonds go immediately before the main chain name. The numbers indicate the first carbon atom in the double/triple bond e.g. 2-hexene 3-methyl-2-hexene 1,3-hexadiene If a compound contains both a double and triple bond, it is an enyne (alphabetical order; ene before yne ). All of these points are included in the rules summary. They are not exceptions.
Recall our functional group priority list: most important carboxylic acids esters amides aldehydes ketones alcohols thiols amines ethers alkenes and alkynes haloalkanes alkanes least important If two groups fall into the same category, and numbering in either direction gives you the same combination of numbers, start at the end closest to the group that comes first in the alphabet. e.g. l l
Name the following unsaturated hydrocarbons using IUPA rules: l l 3 3 3 Which of the molecules above have stereoisomers? What kind of stereoisomer(s)?
Geometric Isomers and Alkenes Recall that cycloalkanes can have cis and trans isomers. Because we cannot rotate about a double bond, alkenes can also have cis and trans isomers: 3 3 3 vs. 3 In the cis isomer, the two largest atoms attached to a double bond are on. In the trans isomer, the two largest atoms are on. Because geometric isomers have different shapes, they also have different properties. e.g. havicine is the molecule responsible for the flavour and smell of black pepper. In the peppercorn, it exists as the cis,cis-isomer. After it is ground, it is slowly converted to the trans,trans-isomer which has less flavour and aroma: 2 2 2 2 N N
Unsaturated fats such as olive oil and canola oil contain cis double bonds. Monounsaturated fats contain one cis double bond while polyunsaturated fats contain several cis double bonds. e.g. oleic acid (in olive oil) ur bo dies need fats containing cis double bonds; however, these fats tend to exist as liquids. To make solid fats (like margarine), a process called hydrogenation is used to saturate the fats with hydrogen: e.g. linoleic acid (in safflower oil) stearic acid (saturated fat) Beca use these fats no longer have the kinks from the cis double bonds, they are no longer good for us. Some companies will partially hydrogenate fats, leaving some of the double bonds. Unfortunately, the hydrogenation process changes the remaining cis double bonds into trans double bonds which also don t have the kinks and are no better for us than saturated fats. e.g. a trans fatty acid
Addition Reactions of Alkenes and Alkynes Because pi bonds are more easily broken than sigma bonds, alkenes and alkynes will tend to react at the double or triple bond. In addition reactions, a pi bond is broken and two new bonds are made, adding two new atoms to the molecule where the pi bond used to be: + A B A B If there are two pi bonds, the addition reaction can happen twice: A A A B Note that the carbon-carbon sigma bond is not touched. ydrogenation ydrogenation is the process of adding molecular hydrogen across a pi bond: e.g. 3 l A B 3 l 3 + ydrogenation will not occur without a catalyst. ommon catalysts include Ni, Pd and Pt. What else do these metals have in common? A B catalyst B l l A B + 3 B B A
The hydrogen molecule is attached to the metal surface. When an alkene (or alkyne) reacts with this hydrogen molecule, both hydrogen atoms are attached to the same side of the newly formed alkane (or alkene): e.g. 3 catalyst 3 catalyst 3 3 3 3 Using normal catalysts, we cannot stop hydrogenation halfway. If we want to make an alkene by hydrogenating an alkyne, we must use a poisoned catalyst ( poisoned Pt or poisoned Pd ). This always makes a alkene. alogenation alogen molecules can also be added across pi bonds. They do not need a catalyst, and the two halogen atoms are attac hed to opposite sides of the newly formed alkane ( or alkene): e.g. 3 3 3 3 + l l 3 l 3 l 3 3
3 3 3 3 3 3 alogenation of an alkyne can be stopped halfway if only one mole of halogen is used for every mole of alkyne. This makes a alkene. omination (halogenation with 2 ) is often used to test for the presence of alkenes or alkynes since 2 is a dark brown liquid but the bromination products are colourless. If the dark brown colour disappears, we know that there was a pi bond to react with the 2. If the colour remains, there was no reactive pi bond. Draw the products of the following reactions: + 2 + 2 Pt 3 3 + 2 l 2 3 3 + 2 2 poisoned Pd
ydrohalogenation It is also possible to add unsymmetrical molecules across pi bonds. When an alkene is reacted with X (X=l, or I), we get a haloalkane. We might predict two potential products for this reaction, depending on which carbon gets the and which gets the l: 3 3 In fact, we only get one product. To see why, we look at the mechanism of reaction (how the reaction works). This reaction takes two steps. First, the electrons in the pi bond attack l, making a carbocation (carbon with positive charge) and a chloride anion: + l 3 3 + l 3 3. l... 3 + 3 Second, the chloride anion attacks the making a new -l bond. 3 + 3 +. l..... - l? or 3 3 3 +. l..... - carbocation, 3 l
Because of this two-step mechanism, we can predict which side of the double bond gets the and which side gets the l (or or I). arbocations are more stable if they have more carbon atoms attached to them: 3 > 2 > 1 > + 3 We always want to make the most stable carbocation possible (Markovnikov s rule). In practice, this means that the hydrogen will attach to the carbon that already has more hydrogens. The halogen will attach to the other carbon, giving the more substituted haloalkane as the major product. What are the major products of the following reactions: + I + l +
ydration We can add water across a pi bond in the presence of an acid catalyst (often written as + ). ydration proceeds according to a similar mechanism to hydrohalogenation except that it uses neutral water instead of chloride anion: 3 3 + + 3 + 3.... 3.. + 3 3 According to this mechanism, why is + a catalyst? + +.... 3 Does Markovnikov s rule still apply? Why? What are the major products of the following reactions? + 2 + + 2 2 +
Polymerisation Addition polymers are made by addition reactions of alkenes to other alkenes. Because these are addition reactions, all of the atoms present in the reactants are also present in the products: l e.g. l l l l l l l heat and pressure Because there are no more pi bonds, the polymers are not reactive and will not break down. This makes them useful as plastic bags, drink bottles, canoes, nonstick pan coating, etc. but means that they are not biodegradable. The individual repeating units that make up a polymer are called monomers. What monomers are used to make the polymers? Saran (polydichloroethylene) following addition F F F F F F F F F F F F Teflon (polytetrafluoroethylene) polystyrene
Aromaticity Aromatic molecules are special kinds of alkenes that are unusually stable. They are so stable that they do not react in the standard alkene reactions. Instead, aromatic molecules have special reactions of their own. So, what makes a molecule aromatic? For our purposes, we will define aromatic molecules as follows: 1. They must contain at least one flat 6-atom ring. 2. The atoms in the ring must all be part of exactly one double bond. This leads to a system of alternating double bonds. e.g. N In this course, we will focus on benzene ( 6 6 ) as its chemistry is representative of all aromatic molecules. We can draw two different structures of benzene both of which are equally valid:
These two structures are resonance structures. They both have all the atoms in the same place, and they both follow the rules for drawing Lewis dot structures. We can imagine the double bonds in benzene moving back and forth (resonating) between the two options so quickly that every bond averages out to a one-and-a-half bond. This is often represented by drawing benzene as follows: What is actually happening is that the p-orbitals of the six atoms in the ring are lined up side-by-side and all six of them share their electrons to hold the molecule together very strongly. Essentially, there are six pi electrons holding the atoms together in a ring. This makes it very difficult to break any of the bonds within the ring. Naming Aromatic ompounds When a benzene ring is the core of the molecule, we use the ring name as the main chain name. The ring is named according to the most important group attached to it, and the carbon attached to that group is numbered 1: 3 N 2 3 benzene toluene aniline phenol anisole
When a benzene ring is attached to a long chain or ring that would be too hard to list as an alkyl group, the chain is used as the main name, and the benzene ring is listed as a phenyl group: phenyl benzyl When a benzene ring * and* 2 are attached to a chain or ring, they are listed as a benzyl group. Name the following compounds containing benzene rings: 2 3 2 l F F
When there are only two groups attached to a benzene rings, we often use a prefix to describe their relationship. If the two groups are on carbons 1 and 2, they are ortho. If the two groups are on carbons 1 and 3, they are meta. If the two groups are on carbons 1 and 4, they are para. l l l l l 1,2-dichlorobenzene 1,3-dichlorobenzene 1, 4-dichlorobenzene = = = ortho-dichlorobenzene (o-dichlorobenzene) meta-dichlorobenzene (m-dichlorobenzene) para-dichlorobenzene (p-dichlorobenzene) These prefixes are only used for benzene rings (not for cyclohexane rings). l Reactions of Aromatic ompounds Even though they are very stable molecules, aromatic compounds can still undergo some reactions. Most of these reactions are specific to aromatic molecules and don t work without an aromatic ring. We will be looking at three kinds of reaction which all fall into the same class. They are all substitution reactions, and the aromatic ring remains intact in all of them it s just the groups attached to the ring that change.
alogenation In halogenation of an aromatic ring, a hydrogen atom is replaced by a halogen atom. This can only take place in the presence of a catalyst like Fel 3 (for chlorination) or Fe 3 (for bromination): 3 Fel 3 + l l and 3 Fe 3 + and Sulfonation In sulfonation of an aromatic ring, a hydrogen atom is replaced by a sulfonic acid group (S 3 ). This is done by reacting the aromatic compound with fuming sulfuric acid ( 2 S 4 that has had S 3 added): 3 + and 2 S 4 S 3
Nitration In nitration of an aromatic ring, a hydrogen atom is replaced by a nitro group (N 2 ). This is done by heating the aromatic compound with a mixture of nitric acid (N 3 ) and sulfuric acid ( 2 S 4 ): 3 2 S 4 + N 3 and Notice that in all three reaction types, the major products are and but not substituted.
Important oncepts from hapter 13 Saturated vs. unsaturated hydrocarbons IUPA rules for naming alkenes IUPA rules for naming alkynes Geometric isomers of alkenes (cis and trans) Addition Reactions: o ydrogenation o alogenation o ydrohalogenation o ydration Markovnikov s rule Addition polymers Aromaticity Reactions of Aromatic Molecules: o alogenation o Sulfonation o Nitration