Part 2 Just Chemistry Department Organic Chemistry 217 Chapter 3 Alkenes And Alkynes كيمياء عضوية ك 217 د. حسين المغيض Dr. Hussein Al-Mughaid
Direct hydration: Addition of H 2 O (Acid-catalyzed hydration) General reaction: catalyst (Nu) (Nu) H OH Addition of water to the double bond forms an alcohol. The addition follows Markovnikov s rule. Uses dilute solutions of H 2 SO 4 or H 3 PO 4 to drive equilibrium toward hydration. Dr. Hussein Al-Mughaid-25
Hydration Mechanism Step 1. Protonation of C=C C C + H + Step2. Nucleophile (H 2 O) attacks the carbocation C H + C H 2 Ȯ... + C H Additional step: C C O H H + C H C H C O H H + C C OH H Dr. Hussein Al-Mughaid-26
Give the products for the following two reactions? alcohol ether Dr. Hussein Al-Mughaid-27
3 Indirect Hydration: Hydroboration Oxidation Gives alcohol with anti-arkovnikov orientation. General reaction: Two steps Stoichiometry : Three moles of alkene can react with each mole of BH 3. Alkylboranes unsymmetrical Dr. Hussein Al-Mughaid-28
Overall reaction: unsymmetrical Give the products for the following two reactions? Dr. Hussein Al-Mughaid-29
4 Hydrogen (H 2 ) can be added across the double bond in a process known as catalytic hydrogenation. The reaction only takes place if a catalyst is used. Example: Q: What alkene would you start with if you wanted to synthesize methylcyclohexane? Dr. Hussein Al-Mughaid-30
5 Addition of Halogens Cl 2, Br 2, and sometimes I 2 add to a double bond to form a vicinal dihalide. Give the product? Dr. Hussein Al-Mughaid-31
6 Oxidation of Alkenes: Two methods 1. Cold, dilute solution of potassium permanganate, KMnO 4 in base. (A chemical test) General reaction: + MnO 2 Give the product? purple Dark brown Dr. Hussein Al-Mughaid-32
2. Ozonolysis Ozone (O 3 ) will oxidatively cleave (break) the double bond to produce aldehydes and ketones. A second step of the ozonolysis is the reduction of the intermediate by zinc or dimethyl sulfide. Zn Then add O Zn, H + Then add O 1. O 3 2. Zn, H + Dr. Hussein Al-Mughaid-33
Solved Problem Ozonolysis reduction of an unknown alkene gives an equimolar mixture of cyclohexanecarbaldehyde and butan-2-one. Determine the structure of the original alkene. Unknown alkene 1. O 3 2. Zn, H + + Solution We can reconstruct the alkene by removing the two oxygen atoms of the carbonyl groups (C=O) and connecting the remaining carbon atoms with a double bond. One uncertainty remains, however: The original alkene might be either of two possible geometric isomers. Dr. Hussein Al-Mughaid-34
Alkynes: Hydrocarbons containing (1s + 2p) General Formula: C n H 2n-2 C n H 2n-2 symmetrical R = R' R = R' Unsymmetrical Dr. Hussein Al-Mughaid-35
Alkynes Nomenclature 1. Find the longest continuous chain that contain the triple bond 2. Proceed from the side that gives the double bond the lowest number 4 3 2 1 Alkane -a +y Alkyne 1 2 3 4 5 6 7 6-bromo-2-methyl-3-heptyne The triple bond determines the numbering Dr. Hussein Al-Mughaid-36
Another one Dr. Hussein Al-Mughaid-37
Give the IUPAC name? Try this one Dr. Hussein Al-Mughaid-38
Acidity of terminal alkynes Terminal alkynes are more acidic than other hydrocarbons due to the higher S character of the sp hybridized carbon. Slightly acidic Acidity increases Dr. Hussein Al-Mughaid-39
How alkynes react like Alkenes Alkynes are nucleophiles and react with electrophiles using electrophilic addition reaction mechanisms! The electrophile adds to the sp carbon of alkyne Nu Addition Product Dr. Hussein Al-Mughaid-40
Addition of H-X Dr. Hussein Al-Mughaid-41
geminal dihalide The 2 Cl,s ended on the same C Can the addition to an alkyne be stopped after the addition of one equivalent of HX? due to the inductive electron withdrawal of the halogen substituent Dr. Hussein Al-Mughaid-42
Dr. Hussein Al-Mughaid-43
Mercuric Ion-Catalyzed Hydration of Alkynes (slow) Mercuric sulfate (MgSO 4 ) in aqueous sulfuric acid adds H OH to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone. Carbonyl group rearrange The hydration is catalyzed by the mercuric ion. In a typical reaction, a mixture of mercuric sulfate in aqueous sulfuric acid is used. The addition produces an intermediate vinyl alcohol (enol) that quickly tautomerizes to the more stable ketone or aldehyde. Dr. Hussein Al-Mughaid-44
Keto Enol Tautomerism Enols (vinyl alcohol = OH on sp 2 carbon) are not stable, and they isomerize to the corresponding aldehyde or ketone in a process known as keto enol tautomerism. sp 2 Dr. Hussein Al-Mughaid-45
Draw the structure of the unknown alkyne? methyl ketone Give the structures of A and B? A + B + H 2 O HgSO 4, H 2 SO 4 O O OH O OH OH A) B) C) D) Dr. Hussein Al-Mughaid-46
3 Addition of Halogens Alkynes has 2 p bonds and therefore, add tow equivalents of the halogen to form a tetrahalide + Dr. Hussein Al-Mughaid-47
4 it is difficult to stop this addition reaction after formation of the alkene Dr. Hussein Al-Mughaid-48
but not impossible - poisoned catalysts (Lindlar catalyst) are partially deactivated metals that are more effective at catalyzing the addition of hydrogen to an alkyne than to an alkene Give the structure of the product? Dr. Hussein Al-Mughaid-49