Lecture Notes: Alkenes and Alkynes PDF

ALKENES ‫ م‬09:38 02/11/2024  Nomenclature For straight chain alkenes, it is the same basic rules as nomenclature of alkanes except change the suffix to "- ene." 1. Find the longest carbon chain that contains the carbon carbon double bond. If you have two ties for longest...

ALKENES ‫ م‬09:38 02/11/2024  Nomenclature For straight chain alkenes, it is the same basic rules as nomenclature of alkanes except change the suffix to "- ene." 1. Find the longest carbon chain that contains the carbon carbon double bond. If you have two ties for longest Carbon chain, and both chains contain a carbon carbon double bond, then identify the most substituted chain. 2. Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond by using the number of the first atom of the double bond as a prefix.  1 2 3 4 1 2 3 4 5 6 H2C CH CH2 CH3 H3C HC CH CH2 CH2 CH3 1-butene 2-hexane (not 3-butene) ( not 4-hexene) 3. Add substituents and their position to the alkene as prefixes. Of course remember to give the lowest numbers possible. And remember to name them in alphabetical order when writing them. 2 3 4 2 3 4 5 6 1 1 2-methyl-2-butene 2,5-dimethyl-2-hexene (not 3-methyl-2-butene) (not 2,5-dimethyl-4-hexene) 2 4 6 3 1 5 2 1 4 Cl 3 5,5-dimethyl-2-hexene 1-chloro-2-butene  4. Number substituted cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference 1 CH3 6 2 1 5 2 H3C 5 4 3 CH3 4 3 1-methylcyclopentene 3,5-dimethylcyclohexene (not 2-methylcyclopentene) (not 4,6-dimethylcyclohexene) 5. Name compounds containing a double bond and alcohol group as alkenols (or cycloalkenol) and give the alcohol carbon the lower number. OH 1 2 6 CH3 CH3 2 1 5 4 3 5 4 3 OH 4-methyl-3-penten-2-ol 2-methyl-2-cyclohexen-1-ol  Synthesis of alkenes A. Dehydration of alcohol using aluminum oxide as a radical Examples Methanol < primary < secondary < tertiary The relative reactivity of alcohols in dehydration reaction  Mechanism 1. the –OH group in the alcohol donates two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion acts as a very good leaving group which leaves to form a carbocation. 2. The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation and form a double bond. Note ❑ Primary alcohols undergo bimolecular elimination (E2 mechanism) ❑ secondary and tertiary alcohols undergo unimolecular elimination E1 mechanism) ‫ م‬09:38 02/11/2024 6  E2 mechanism (1ry alcohol) 1) Oxygen donates two electrons to a proton from sulfuric acid H2SO4, forming an alkyloxonium ion. 2) The nucleophile HSO4– back-side attacks one adjacent hydrogen and the alkyloxonium ion leaves in a concerted process, making a double bond. ‫ م‬09:38 02/11/2024 7  E1 mechanism (2nd & 3ry alcohol) 1) (OH) protonates to form alkyloxonium ions. 2) the ion leaves first and forms a carbocation as the reaction intermediate. 3) The water molecule (which is a stronger base than the HSO4- ion) then abstracts a proton from an adjacent carbon, forming a double bond. ‫ م‬09:38 02/11/2024 8 ‫‪ 09:38 02/11/2024‬م‬ ‫‪9‬‬ ❖Notes: The Red arrows show formation of the more substituted 2- butene, while the Blue arrows show formation of the less substituted 1-butene. More substituted alkenes are more stable than less substituted alkenes, and trans alkenes are more stable than cis alkenes. ‫ م‬09:38 02/11/2024 10  B) From Alkynes (stereoselective synthesis) If the cis-alkene hydrogenation in the presence of Lindar’s catalyst (palladium deposited on calcium carbonate) or hydroboration followed by hydrolysis If the Trans-alkene Reduction of the alkyne by sodium metal in liquid ammonia gives the trans-alkene ‫ م‬09:38 02/11/2024 11  C- Dehydrohalogenation of alkyl halides (haloalkanes): With sodium ethoxide. ‫ م‬09:38 02/11/2024 12  Reactions of Alkenes Addition Reaction: (Exothermic reaction) 1- Addition of Strong Bronsted Acids a) Strong Bronsted acids such as HCl, HBr, HI & H2SO4 b) When added to the C=C of alkenes, new covalent bonds are formed to hydrogen and to the conjugate base of the acid ‫ م‬09:38 02/11/2024 13 Note…. Weak Bronsted acids such as water (pKa = 15.7) and acetic acid (pKa = 4.75) do not normally add to alkenes. However, the addition of a strong acid serves to catalyze the addition of water, and in this way alcohols may be prepared from alkenes CH2=CH2 + H3O(+) ——> HCH2–CH2OH + H(+) ‫ م‬09:38 02/11/2024 14 When addition reactions to such unsymmetrical alkenes are carried out, we find that one of the two possible constitutionally isomeric products is formed preferentially. Selectivity of this sort is termed Regioselectivity Any process that favors bond formation at a particular atom over other possible atoms. ‫ م‬09:38 02/11/2024 15  2-Rearrangement of Carbocations Rearrangement of the initially formed 2º-carbocation to a 3º- carbocation by a 1,2-shift of a methyl group (a shift of a neighboring alkyl group or hydrogen) ‫ م‬09:38 02/11/2024 16 3-Addition of Lewis Acids (Electrophilic Reagents) Lewis acids like the halogens, boron hydrides and certain transition metal ions are able to bond to the alkene pi- electrons, and the resulting complexes rearrange or are attacked by nucleophiles to give addition products ‫ م‬09:38 02/11/2024 17  4- Oxidation of alkenes Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized. Several oxidizing agents, e.g. KMnO4, K2Cr2O7, O3, and peroxy acids may be used. (a) Oxidation by potassium permanganate to glycol OH, H2O CH2 CH2 + KMnO4 CH2 CH2 cold OH OH Ethene 1,2-ethanediol (ethylene glycol) 02/11/2024 18 ‫ م‬09:38  (b) Oxidation by ozone to ozonoid(ozonolysis of alkenes). H3C CH3 H3C CH3 (1) O3, CH2Cl2, - 78oC C C C O + O C H3C H (2) Zn/ HOAc H3C H 02/11/2024 19 ‫ م‬09:38  Alkynes ❑The general molecular formula for an acyclic (noncyclic) alkyne is CnH2n-2 ❑Each carbon is sp hybridized, so each has two sp orbitals and two p orbitals ❖Synthesis of Alkynes by Double Elimination (E2 mechanis) ✓from dihaloalkanes through dehydrohalogenation using alkoxide bases ✓if we apply this concept using 2 halides on vicinal or geminal carbons, the E2 reaction will take place twice resulting in the formation of 2  bonds and thus an Alkyne ‫ م‬09:38 02/11/2024 20  Reactions of Alkynes An alkyne is an electron rich molecule. In other words, it is a nucleophile and consequently it will react with electrophiles The mechanism of the additions is presumably the same as with the alkenes, the initial step involving reaction between the nucleophilic unsaturated carbon atom and the electrophilic reagent atom ‫ م‬09:38 02/11/2024 21 1.Addition by Electrophilic Reagents o The reactions are even more exothermic than the additions to alkenes o It takes place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. o The products of these additions are themselves substituted alkenes and can therefore undergo further addition ‫ م‬09:38 02/11/2024 22  2-Hydration of Alkynes and Tautomerism ❑As with alkenes, the addition of water to alkynes requires a strong acid, usually sulfuric acid, and is facilitated by mercuric sulfate ❑However, unlike the additions to double bonds which give alcohol products, addition of water to alkynes gives ketone products ❑Due to enol-keto tautomerization, The initial product from the addition of water to an alkyne is an enol (a compound having a hydroxyl substituent attached to a double-bond), and this immediately rearranges to the more stable keto tautomer ‫ م‬09:38 02/11/2024 23  3-Oxidations Reactions of alkynes with oxidizing agents such as potassium permanganate and ozone usually result in cleavage of the triple-bond to give carboxylic acid products RC≡CR' + [O] ——> RCO2H + R'CO2H ‫ م‬09:38 02/11/2024 24