Hydrocarbon Reactions: Addition, Substitution and Elimination Today’s Objectives:
1) Define, illustrate and provide examples of simple addition, substituion, elimination and esterification (condensation)
2) Predict products and write and interpret balanced equations for the above reations
Section 10.2,10.3 and 10.4 (pg. 419-444) Hydrocarbon Reactions
1. Addition
alkenes and alkynes + H2(g) alkanes (hydrogenation)
alkenes and alkynes + HX (or X2) organic halides 2. Substitution
alkanes and aromatics + X2 organic halides 3. Elimination alcohols alkenes + water (dehydration) organic halides + OH- alkenes + halide ion + water
4. Esterification (already covered) carboxylic acid + alcohol ester + water 1. Addition Reactions: reaction of alkenes and alkynes with hydrogen gas, a halogen compound, or a hydrogen halide compound. Addition reactions usually occur in the presence of a catalyst
a) Addition with H2(g) (also called hydrogenation)
1. Addition Reactions: reaction of alkenes and alkynes with hydrogen
gas, a halogen compound, or a hydrogen halide compound
b) Addition of a halogen
1,2-dichloroethane is a useful solvent and is used to produce chloroethene, the monomer used to make PVC. See Pg. 417 equation
Since addition reactions involving multiple bonds are very rapid, the alkene product 1,2-dibromoethene can easily undergo a second addition step to produce 1,1,2,2-tetrabromomethane.
Excess bromine promotes this second step. 1. Addition Reactions: reaction of alkenes and alkynes with hydrogen gas, a halogen compound, or a hydrogen halide compound
c) Addition of an HX (hydrogen halide) molecule Show both possible isomers when predicting the products
The addition of hydrogen halides (HF, HCl, HBr or HI) to unsaturated compounds can produce structural isomers, since the hydrogen halide molecules can add in different orientations. However, the type of isomers produced are not always equal.
For example only a tiny amount of 1-chloropropane is produced compared to 2-chloropropane in the above reaction. Practice Addition Reactions
For each of the following questions, draw condensed structural diagrams and name all products
1. ethene + bromine 1,2-dibromoethane
2. hydrogen chloride + ethene chloroethane
3. 3-methylbut-1-yne + excess hydrogen
2-methylbutane __
2. Substitution Reactions – breaking of a C-H bond in an alkane or an aromatic ring and replacing it with another atom or group of atoms
Usually occur slowly at room temperature, so light may be necessary as a catalyst
Often substitutes a halogen for a hydrogen No change in saturation
Propane contains hydrogen atoms bonded to end carbons and the middle carbon atom, so two different products (isomers) are formed, in unequal proportions 2. Substitution Reactions – breaking of a C-H bond in an alkane or an aromatic ring and replacing it with another atom or group of atoms
Benzene rings are stable, like alkanes, so they react slowly with halogens, even in the presence of light.
Would all three isomers be created equally? NO
As with alkanes, further substitution can occur in benzene rings, until all hydrogen atoms are replaced by halogen atoms (in the presence of excess halogens) Practice Substitution Reactions
For each of the following questions, draw condensed structural diagrams and name all products
1. propane + fluorine 1-fluoropropane + 2-fluoropropane + hydrogen fluoride F - F H - F
2. ethane + chlorine chloroethane + hydrogen chloride
3. Elimination Reactions – involves eliminating atoms or groups of atoms from adjacent carbon atoms; decreases the level of saturation a) Alkane cracked into an alkene (uses high temperatures)
b) Alcohol is reacted with a catalyst to produce an alkene and water (dehydration – removes a water molecule from the alcohol)
a) Alkyl halide reacts with a hydroxide ion (OH-) to produce an alkene (dehydrohalogenation – removes a hydrogen and halogen atom) Practice Elimination Reactions
Write a structural formula equation for the preparation of but- 2-ene from chlorobutane, in the presence of a strong base
Write a structural formula equation for the preparation of but- 2-ene from butan-2-ol
Reaction Complete Addition Elimination Substitution Esteri- type Combustion fication Reactants Hydrocarbon* ene or yne + alcohol or alkane or alcohol + HOH, X , HX, alkyl halide aromatic + X carboxylic + O2 2 2 or H2 acid
Products CO2(g) + alcohol, alkyl alkene + HOH alkyl halide ester + H2O(g) or (l) halide*, or HX water alkane Other triple or single to slow; needs acid double to double uv light catalyst single needed Other more bonds* fewer bonds* Today’s homework
Pg. 422 #7,8 (Addition and Substitution)
Pg. 433 #18-19 (Elimination)
What is coming up tomorrow? Polymerization STS Connections