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Chemistry of polyfunction 1. Types of carbon atom Dr. Dina akhotmah-232 2
Classification of multiple bonds of polyunsaturated compounds Dr. Dina akhotmah-232 3
Organic chemistry, solomons, 10 th edition, page 599 Many hydrocarbons are known that contain more than one double or triple bond: a) Hydrocarbons with two double bonds is called an alkadiene (or dienes) b) three double bonds is called an alkatriene (or trienes), and so on c) A hydrocarbon with two triple bonds is called an alkadiyne d) a hydrocarbon with a double and triple bond is called an alkenyne The following examples explain how specific compounds are named. Dr. Dina akhotmah-232 4
Allylic substitution and allyl radical Reference organic chemistry, solomons, chapter 13, page 585- when propene for example reacts with chlorine or bromine at low temperatures, an addition reaction of halogen to the double bond is take place However, when propene for example reacts with chlorine or bromine at very high temperatures or low concentration of the halogen, the reaction that occurs is a substitution Dr. Dina akhotmah-232 5
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Why allyl product is formed? Dr. Dina akhotmah-232 7
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The resonance description of the allyl radical The allyl compound break either hemolytic producing free radical Heterolytic producing cation or anion All these product resonate witch explain its stability, high reactivity and the possibility to form two products. For an example: Why B is the major product? Think back to Zaitsev s rule the more substituted an alkene is, the more stable it is Dr. Dina akhotmah-232 10
H C C H C H 3 N B S, h e a t C C l 4 H H C A C H d i r e c t p r o d u c t C H 2 B r H C C H 2 C B r B a l l y l i c r e a r r a n g m e n t p r o d u c t Dr. Dina akhotmah-232 11
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Organic Chemistry, solomons, 10 th edition, page 602 Dr. Dina akhotmah-232 13
Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes, these stability can be seen in an analysis of the heat of hydrogenation. For an example, Dr. Dina akhotmah-232 14
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Electrophilic Attack on Conjugated Dienes 1,4 Addition Conjugated dienes undergo both 1,2 and 1,4 addition through an allylic intermediate. for example, 1,3-butadiene react with one molar equivalent of hydrogen chloride to give two products, 3-chloro-1-butene and 1-chloro-2-butadiene. Why? Dr. Dina akhotmah-232 16
The addition of HX undergo by allylic intermediate, the mechanism will give better understanding for this result Reaction Mechanism Dr. Dina akhotmah-232 17
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1.2 addition 1.4 addition Dr. Dina akhotmah-232 20
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(Page 625) Dr. Dina akhotmah-232 22
Preparation of conjugated diene Dienes can be prepared by elimination reactions of unsaturated alcohols and alkyl halides. The outcome of eliminations typically favors the more stable product, Since conjugated dienes are more stable than isolated dienes, the formation of the conjugated diene is usually favored over the isolated diene unless the structure prevents the formation of the conjugated system. H H O R H R H 2 S O 4 K O H h e a t h e a t c o n j u g a t e d d i e n e s H H B R r What would the products of the following sequences be? Why? a) 1-butene reacted with N-bromosuccinimide (NBS) then treated with KOH / heat b) cyclohexene reacted with Br 2 / CH2Cl2 then with KOH / heat Dr. Dina akhotmah-232 23
a) H H H 2 C C C CH 3 NBS heat, Cl H 4 1-butene Br KOH, heat -HBr morestablediene b) Br Br 2 /Cl 4 KOH, heat -HBr Br KOH, heat -HBr Br Dr. Dina akhotmah-232 24
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Class exercises Dr. Dina akhotmah-232 27
Preparation of isoprene O H C C H s t r o n g b a s e O H H O H C + + H 3 C C H 3 C C H H 3 C C C H 3 C C H H 2 / P d H 2 C A l 2 O 3 - H 2 O C C H 3 C H C H 2 p o l y m e r i z a t i o n H 2 C C H C C H 2 C H 3 i s o p r e n e Chemical structure of cis polyisoprene the main constituent of natural rubber Dr. Dina akhotmah-232 28
Terpenes Terpenes is made of isoprene unit (2-methyl-1,3-butadiene) linked in a head-to-tail fashion. (isoprene is a building block of terpenes). For example, Monoterpenes are consists of two isoprene units, They are volatile natural products found in higher plants as essential oils and are widely used in perfumery and flavouring industries. For example, Lemon oil and Myrcene Dr. Dina akhotmah-232 29
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Solomon's, page 616 Dr. Dina akhotmah-232 31
What Is the Diels-Alder Reaction? Heat 200 o C The reaction occurs via a concerted mechanism, meaning that it takes place all at once, with no isolatable intermediates. Dr. Dina akhotmah-232 32
Diels-alder reaction is favoured by presence of : a) Electron-releasing groups in the diene (e.g. -R, -OR) b) Electron-withdrawing groups in the dienophile, for example Maleic anhydride The easiest way to predict Diels-Alder products is to number the six carbons that make up the cyclohexene ring. Note: this is not a nomenclature numbering Dr. Dina akhotmah-232 33
The Diels-Alder is Stereospecific The Diels-Alder is a stereospecific reaction, because the stereochemistry of the dienophile is maintained in the cyclohexene product: a trans dienophile yields in the trans product, and a cis dienophile yields the cis product. What Can and Can t Undergo Diels-Alder? Only conjugated dienes in the s-cis conformation can undergo Diels-Alder reactions. If a diene is not conjugated, or can t be in the cis conformation, then it can t undergo a Diels- Alder reaction. Dienes that are located in rings can t rotate, and so are stuck in either an s-cis or s-trans conformation. But acyclic dienes can rotate into the s-cis conformation, as long as their substituents aren t too big/bulky to prevent easy rotation. Dr. Dina akhotmah-232 34
Retro Diels-Alder The Diels-Alder reaction is reversible. Going from cyclohexene to a diene and dienophile is called a retro Diels-Alder reaction. It s also a very common to be asked what starting materials produced this Diels-Alder product? The best strategy for determining retro Diels-Alder questions is also to number the cyclohexene ring. Below are some examples. Dr. Dina akhotmah-232 35
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Multiple Choice Quiz For a diene to undergo Diels-Alder reaction it must: a) be substituted with electron-withdawing groups. b) be able to adopt an s-trans conformation c) be substituted with electron-donating groups d) be able to adopt the s-cis conformation. What is the major product of the following reaction: None of the Above More questions and information's on chemheler.com Dr. Dina akhotmah-232 37