Goals for the Day… Addition Reactions of Alkenes & Alkynes

Addition Reactions In addition reactions, a double or triple bond is easily broken, providing electrons for new bonds. reactants add to the carbon atoms in double or triple bonds.

Addition Reactions There are different addition reactions as shown in Table 12.2. We will look at each of these separately…

Hydrogenation In hydrogenation, hydrogen atoms add to the carbon atoms of a double or triple bond. a catalyst, such as Pt, Pd or Ni, is used to speed up the reaction.

You may also see the reaction written with the reagents over the arrows & with condensed notation… cis addition of hydrogen to the double bond

Hydrogenation of Oils Adding H2 to double bonds in vegetable oils produces compounds with higher melting points. solids at room temperature, such as margarine, soft margarine, and shortening.

Learning Check Draw the condensed structural formula or line formula of the product in your composition book… ?

Solution Remember that each carbon gains a hydrogen atom in this reaction…they may or may not be shown.

Learning Check Draw the condensed structural formula or line formula of the product in your composition book…

Solution Again, various ways to show the product…

Learning Check Write an equation for the hydrogenation of 1-butene using a platinum catalyst.

Learning Check Write an equation for the hydrogenation of 1-butene using a platinum catalyst. … 1-butene is

Learning Check And adding hydrogens will look something like this…

Answer The product will be butane…

Halogenation In a halogenation reaction, halogen atoms, such as chlorine or bromine, add to the carbon atoms of a double or triple bond.

Halogenation Halogenation reactions of alkenes and alkynes occur rapidly without a catalyst. Note that each carbon of the double bond gains a halogen (with an alkyne each gets two)…

Testing for Unsaturation Br2 When bromine (Br2) is added to an alkane, the red color of bromine persists. to an alkene or alkyne, the red color of bromine disappears immediately. Br2

Learning Check Draw the condensed structural formula of the product of each the following addition reactions. ? 2. Br2(l) ?

Solution Draw the condensed structural formula of the product of each the following addition reactions. 2. Br2(l)

Try to predict the starting material… (there is a hint on the next slide)

We need an alkene, but which one?…

Since the bromines are on carbons 2 & 3, the double bond was there too

If you started with “a” you would have 1,2 dibromo pentane

Hydrohalogenation In hydrohalogenation, the atoms of a hydrogen halide, such as HCl or HBr, adds to the carbon atoms of a double or triple bond without a catalyst.

Hydrohalogenation Hydrohalogenation reactions occur in two steps.

Stability of carbocations… So the more carbons that are attached to the center (carbocation) carbon, the more stable it is…

Markovnikov’s Rule In assymetrical alkenes, the H in HX adds to the carbon in the double bond that has the greater number of H atoms.

Which is the preferred carbocation? Hint on the next page…

Which is more stable? Adding H here gives the red carbocation Adding H here gives the green carbocation…

The green path is more stable Adding H here gives the green carbocation…it is more stable! 

What is the end result? Bromine can now add, and you get 2-bromo propane as the main product… Let’s try more! 

Learning Check Predict the product for the following reaction of 1-pentene with HCl.

Solution H adds to the carbon that has more hydrogen atoms attached.

Hydration In hydration, a strong acid (H+) catalyst is required. water (HOH) adds to a double bond. an −H atom bonds to one C in the double C═C bond. an −OH bonds to the other C.

Hydration When hydration occurs with a double bond that has an unequal number of H atoms, the −H atom bonds to the C in the double C═C bond that has more hydrogen atoms. the −OH bonds to the C in the double C═C bond that has the fewer hydrogen atoms.

Learning Check Draw the condensed structural formula of the product of the following hydration reaction in your composition books….

Solution Remember to follow Markovnikov’s Rule! 

Learning Check Draw the condensed structural formula of the product of each the following reactions.

Solution Draw the condensed structural formula of the product of each the following reactions.

Last one for the day… Try to draw the product in your composition books…(there is a hint on the next slide).

Hint… Remember you only have two choices of where to add the OH. (it can only add to the carbons where the double bond is). Would the OH add to carbon 2 or carbon 3?

One way of drawing the product… In this case the OH will add to carbon 2 since it has more carbons attached to it. (the hydrogen of the water adds to carbon 3).