Transcript Unit 1
UNIT 2 • Conjugated systems •Stability •Reactions • Aromatic cpds and their Chemistry Simple alkenes The relative stability of an alkene can be determined by comparing the heats of hydrogenation. The more highly substituted alkene is lower in energy. The ∆H values are additive and can be used to estimate the heat of reaction for the hydrogenation of other alkenes. What is the ∆ H for the hydrogenation of penta-1,4-diene? Dienes There are three types of dienes. Isolated dienes (at least one CH2 between the sp2 hybrid carbons) Conjugated dienes (alternating single and double bonds) Cumultive dienes Conjugated alkenes The ∆H for the hydrogenation of a conjugated diene is significantly lower than what is predicted. The conjugated diene is more stable than the un-conjugated diene by 17 KJ. This difference in energy levels (predicted verses found) is called the resonance energy. Conjugated alkenes MO diagram for Conjugated alkenes MO diagram for ethylene. Note that there are only electrons in the bonding MO. MO diagram for Conjugated alkenes Simple alkenes We have seen that reactions occur preferentially at the allylic position of an alkene. CH3CH CHCH2CH3 allylic positions The reason for the increase in reactivity at the allylic position is that the reactive intermediates formed at the allylic position are resonance stabilized. CH2 CHCHCH3 CH2CH CHCH3 Allylic substrates can be used in either Sn1 or Sn2 reactions. Note that whenever there is a reactive intermediate that is resonance stabilized there is the possibility of two or more products. Resonance stabilization of reactive intermediates Draw the other resonance structures for the last three cations. Draw the resonance structures of the corresponding free radicals and anions. Allylic halides are also excellent substrates for SN2 reactions (involve strong nucleophiles) due to overlap of the temporary p orbital in the transition state with adjacent p orbitals of the double bond. Reactions of alkenes Alkenes can under go either addition reactions or substitution reactions in the allylic position. Br2 CH2 CHCH3 CH2 CHCH3 Br Br NBS CH2 CHCH3 CH2 CHCH2 Br What products would be formed from the addition of HBr? The addition of Br2/H2O? What is NBS? What about Sn1 and Sn2 reactions? Reactions of alkenes With conjugated alkenes the reactive site may be delocalized over two or more atoms in the molecule. Reactions of conjugated alkenes When adding one equivalent of reagent you will obtain two addition products. What would happen with a excess of reagent? Reactions of conjugated alkenes To understand the above observation we must consider both the kinetics and the thermodynamics of the reaction. Reactions of conjugated alkenes What happens to the kinetics of the two reactions at low temperature? Why is the lowest energy product formed at higher temperature? Diels-Alder Reaction The Diels-Alder reaction is a concerted cycloaddition reaction. Also called a [4+2] cycloaddition. The reagents are an electron rich diene and an electron deficient alkene (dienophile). Diels-Alder Reaction The dienophile can also be an electron deficient alkyne. Diels-Alder Reaction Two possible planar conformations for conjugated dienes: H H = s-trans s-cis H H The rotational barrier around the C2-C3 bond is about 5 kcal/mol. At room temperature there is essentially free rotation about this bond. The s-cis conformation is a stereo chemical requirement for the Diels-Alder reactions. Diels-Alder Reaction Things to consideration when drawing the product of a Diels-Alder reaction. 1. Stereochemistry of the diene. 2. Stereochemistry of the product. 3. Orientation of the diene to the dienophile (where are the substituents going to be on the ring). Diels-Alder Reaction Things to consideration when drawing the product of a Diels-Alder reaction. 1. Stereochemistry of the diene. 2. Stereochemistry of the product. 3. Orientation of the diene to the dienophile (where are the substituents going to be on the ring). Diels-Alder Reaction When there is a pi bond in the electron withdrawing group of the dienophile the group is generally going to be in the endo position in the product. This is called the endo rule. We will always use the above orientation of the diene and dienophile when predicting the product of a Diels-Alder reaction. Diels-Alder Reaction H With the dienophile coming from the bottom the following the stereochemistries will be observed. The substituent groups pointing “inside” the diene get pushed up, groups pointing “outside” the diene get pushed down, and electron withdrawing groups end up in down positions whenever possible. W R H R W + R W W H R H R W H R H W + H W W R H R R W H R H W + H R W W R H Diels-Alder Reaction Diels-Alder Reaction Diels-Alder Reaction Predict products for the following reactions: + COOCH3 C C H OMe H3 C CN + Ph COOH O + Diels-Alder Reaction Predict products for the following reactions: Diels-Alder Reaction Draw the structures of the dienes and dienophiles used to make the following Diels-Alder adducts: COOC2 H5 H3 C H3 C CH3O CN CN CH3 O O