C Organic 1 Lecture 3 PDF

Nahda University Faculty of Pharmacy Pharmaceutical Chemistry Department Pharmaceutical Organic Chemistry I CPO1101 PharmD Clinical Lecture (3) Alkynes...

Nahda University Faculty of Pharmacy Pharmaceutical Chemistry Department Pharmaceutical Organic Chemistry I CPO1101 PharmD Clinical Lecture (3) Alkynes 1 Alkynes CnH2n-2 2  Alkynes  Hydrocarbons that contain carbon-carbon triple bonds.  Acetylene, the simplest alkyne H-C≡C-H.  The general formula for the Alkyne is: CnH2n-2  Recall that the triple bond consists of a σ bond and 2 π bond.  Each carbon is sp hybridized and planar, with bond angles of approximately 180 ° 3  1) Physical properties of alkynes  Similar to alkane and alkene.  Alkynes are only very slightly soluble in water although they are more soluble than alkenes and alkanes, due to extra π bond. 4  2) Nomenclature of Alkynes Step 1: Name the parent hydrocarbon, Name the parent hydrocarbon, find the longest carbon chain containing the triple bond, and name it using the greek prefix representing the number of the chain & suffix –yne instead of –ane in case of alkanes. Step 2: Number the carbon atoms in the chain, Number the carbon atoms in the chain, begin at the end nearer to the triple bond, if the triple bond is equidistant from the two ends, begin at the end nearer to the first branching point. Step 3: Write the full name, Number the substituents according to their positions in the chain, & list them alphabetically. Indicate the position of the triple bond by giving the number of the first carbon of the triple bond & placing that number directly before the parent name. 5  2) Nomenclature of Alkynes  If the compound contains both double and triple bonds double bond takes priority unless the least possible numbering is given to Alkyne.  Multiple triple bonds are: diynes, triynes, etc…  Number nearest a multiple bond (either double or triple), If both are equidistant from both ends then double bond will take lower number than triple. 6 7 8 9 10  3) Preparation of alkynes (I) From Vicinal or Geminal Preparation of alkynes dihalides by double dehydrohalogenation 11  3) Preparation of alkynes I) From Vicinal or Geminal dihalides by double dehydrohalogenation:  Treatment of a 1,2-dihalidoalkane with KOH or NaOH produces a two-fold elimination of HX (double dehydrohalogenation).  Remember that, Vicinal dihalides are available from addition of bromine or chlorine to an alkene. 12  4) Reactions of alkynes (I) Catalytic a) Addition of Hydrogen Hydrogenation Halides (HX) (II) Electrophilic b) Addition of Water Addition Reaction (Hydration) Reactions of alkynes (III) Reaction at acidic hydrogen (Terminal c) Addition of Halogen Alkynes) (IV) Oxidative Cleavage 14  4) Reactions of alkynes I) Catalytic Hydrogenation: 15  4) Reactions of alkynes II) Electrophilic Addition reaction:  Although the pi electron density in a triple bond is higher than that in a double bond, alkynes are less reactive than alkenes towards electrophilic reagents. 1- alkyne pi electrons are contained in a shorter more compact molecular orbital between 2 sp hybridized carbon atoms and are thus perhaps less accessible to an attacking electrophile. 2- Vinyl (Alkenyl) cations are less stable than alkyl cations. 16  4) Reactions of alkynes II) Electrophilic Addition reaction: a) Addition of Hydrogen Halides (HX): According to Markovnikov’s rule (the positive portion adds to the carbon that has more Hs) “the rich gets richer” 17  4) Reactions of alkynes II) Electrophilic Addition reaction: b) Addition of Water (Hydration): Alkynes add water when the reaction is catalyzed by strong acids (sulphuric acid) and mercuric sulphate to yield aldehyde or a ketone. Keto-enol Tautomerism  Enols rearrange to the isomeric keto form by the rapid transfer of a proton from the hydroxyl to the alkene carbon.  The keto form is usually more stable compared to the enol. 18  4) Reactions of alkynes II) Electrophilic Addition reaction: b) Addition of Water (Hydration): 19  4) Reactions of alkynes II) Electrophilic Addition reaction: c) Addition of Halogen: If 1 mole of halogen (Br2 or Cl2) adds to 1 mole of an alkyne, the product is a dihaloalkene. If 2 moles of halogen add to 1 mole of an alkyne, tetrahalide will be the product. 20  4) Reactions of alkynes III) Reaction at acidic hydrogen (Terminal Alkynes):  Terminal alkynes and acetylene are fairly acidic.  Why H in acetylene is acidic? This acidity is due to the ability of the negative charge in the acetylide conjugate base to be stabilized as a result of the high s character of the sp orbital, in which the electron pair resides.  As a result, a proton is more easily removed from the carbon as electrons flow more willingly to a more electronegative atom. 21  4) Reactions of alkynes III) Reaction at acidic hydrogen (Terminal Alkynes):  Conversion of smaller alkynes into larger ones is achieved by reaction of metal acetylides with a suitable 1° alkyl halide (SN2)?!. 22  4) Reactions of alkynes III) Reaction at acidic hydrogen (Terminal Alkynes):  Acetylene and terminal alkynes also form metal derivatives with silver nitrate (AgNO3).  How to differentiate between but-1-yne & but-2-yne?!! 23  4) Reactions of alkenes IV) Oxidative cleavage of Alkynes: Treating alkynes with ozone (O3) or strong oxidizing agents (KMnO4) leads to cleavage of the triple bond to produce carboxylic acids. 24 How to convert pent-1-yne to octane?! 25  Q1: Give the IUPAC nomenclature?! ………………………………………………………. ………………………………………………………. ………………………………………………………. ………………………………………………………. 26 Q2: Draw the structures for the following names?! i) 3-methyl-1-penten-4-yne ii) 3-Hexene-1 –yne iii) 6,6-dimethylhept-3-yne 27 Q3: Complete the following equations?! 28 29 30 Q4: The following are incorrect names give the correct ones:- i) 4-Methyl-2-hexen-5-yne ii) 3-Ethyl-6,6-dimethylcyclohexene iii) 2-Methyl-3-butane iv) 2-Methyl-3-pentene 31 Q5: Convert:- i) Acetylene into acetone ii) 2-Bromopropane into 1,2-dibromopropane iii) Acetylene into 1-butyne iv) 2-bromobutane into acetic acid v) 2-Bromopropane into propyne 32 Q6: How to differentiate between:- i) Cyclobutane & n-butane ii) n-Hexane & 1-hexyne iii) n-Hexane & 1-hexene iv) 1-butyne & 2-butyne v) 1-butene & 2-butene 33 34