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Yne Disubstituted Alkynes, RC≡C-R', Are Described As "Internal"

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Yne Disubstituted Alkynes, RC≡C-R', Are Described As Nomenclature: Functional group suffix = -yne Disubstituted alkynes, R-C≡C-R', are described as "internal" alkynes Monosubstituted alkynes, R-C≡C-H, are described as "terminal" alkynes. ethyne (or acetylene) propyne terminal terminal 1-butyne 2-butyne terminal internal Classify each of the following as an internal or a terminal alkyne: (a) 1-hexyne (c) cyclooctyne (b) 3-octyne (d) propyne Stability: • As with alkenes, the more highly substituted internal alkynes are more stable. • By comparing thermodynamic data of alkynes and alkenes, it can be seen that the "extra" p bond in an alkyne is weaker than the alkene p bond: DHh 1-hexyne = 290 kJ/mol (69.2 kcal/mol) vs 1-hexene = 126 kJ/mol (30.2 kcal/mol) So C≡C to C=C = 164 kJ/mol (39 kcal/mol) while C=C to C-C = 126 kJ/mol (30.2 kcal/mol) Therefore the "extra" p bond is 38 kJ/mol (8.8 kcal/mol) weaker that an alkene p bond. Rank 1-hexyne and 3-hexyne for each of the following properties: (a) heat of hydrogenation (c) heat of formation (b) heat of combustion (d) stability Structure: • The alkyne functional group consists of two sp hybridised C atoms bonded to each other via one s and two p bonds. • The 2 p bonds are produced by the side-to-side overlap of the two pairs of p-orbitals not utilised in the hybrids. • The substituents are attached to the C≡C via sigma bonds. • The 2 C of the C≡C and the 2 atoms attached directly to the C≡C are linear. • Since alkynes are linear, they cannot exist as cis- / trans- isomers. Combined p molecular The two separate perpendicular p molecular orbitals orbitals Physical Properties: As with hydrocarbons in general, alkynes are non-polar and are insoluble in water but soluble in non-polar organic solvents. Reactivity: • The p bonds are a region of high electron density (red) so alkynes are typically nucleophiles. • Alkynes typically undergo addition reactions in which one or both of the p-bonds are converted to new s bonds. • Terminal alkynes, R-C≡C-H, are quite acidic (indicated by blue) for hydrocarbons, pKa = 26 • Deprotonation of a terminal acetylene gives an acetylide ion.The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes. Acidity of Terminal Alkynes Summary Terminal alkynes are unusual for simple hydrocarbons in that they can be deprotonated (pKa = 26) using an appropriate base (typically NaNH2, pKa = 36) to generate a carbanion which can function as a C centered nucleophile and so allow for the formation of new C-C bonds by reacting with C centered electrophiles (such as alkyl halides). In order to appreciate what makes the terminal alkyne more acidic than most other hydrocarbons, we should look at the stability of the conjugate base (i.e. the carboanion). For each type of carbanion shown, the nature of the hybrid orbital containing the electron pair is important. Increased s character (sp = 50%, sp2 = 33% and sp3 = 25%) implies that the alkyne sp orbital is closer to the nucleus and so there is greater electrostatic stabilisation of the electron pair. Therefore the conjugate base of the alkyne is the most stable and the most readily formed. However the terminal alkyne C-H bond is not strongly acidic and a strong base, - such as the amide ion, NH2 , is required to form the carbanion. Could you use a base such as NaOH or NaOEt for this reaction ? Related reactions • Alkylation of terminal alkynes Alkylation of Alkynes Reaction Type: Acid / Base and Nucleophilic Substitution Summary • Terminal alkynes are unusual for simple hydrocarbons in that they can be deprotonated (pKa = 26) using an appropriate base (typically NaNH2, pKa = 36) to generate a carbanion. • The acetylide carbanion is a good C nucleophile and can undergo o o nucleophilic substitution reactions (usually SN2) with 1 or 2 alkyl halides (Cl, Br or I) which have electrophilic C to produce an alkyne. o • 3 alkyl halides are more likely to undergo elimination. • One or both of the terminal H atoms in ethylene (acetylene) H-C≡C-H can be substituted providing access to monosubstituted (R-C≡C-H) and symmetrical (R = R') or unsymmetrical (R ¹ R') disubstituted alkynes R- C≡C-R' • Note that since the product is also an alkyne, it can also undergo the other reactions is this chapter. MECHANISM FOR ALKYLATION OF ALKYNES Step 1: An acid / base reaction. The amide ion acts as a base removing the acidic terminal H to generate the acetylide ion, a carbon nucleophile. Step 2: A nucleophilic substitution reaction. The carbanion reacts with the electrophilic carbon in the alkyl halide with loss of the leaving group, forming a new C-C bond. - What is the product of the reactions of CH3-C≡C with each of the following: (a) 2-bromopropane (d) ethanol (b) 1-iodooctane (e) ethyl tosylate (c) (R)-2-bromohexane (f) bromobenzene Related reactions: • Acidity of teminal alkynes • Other substitution reactions of alkyl halides • Alkylation of enolates Preparation of Alkynes via Elimination reactions Reaction Type: Elimination (E2) Summary • The two p bonds of an alkyne can be formed using two consecutive elimination reactions. • The leaving groups are usually halides (esp. Br or Cl) • Reagent : usually NaNH2 (a strong base) • Either geminal (1,1-) or vicinal (1,2-) dihalides can be used. • Since 1,2-dihalides can be prepared by addition of X2 to an alkene, an alkene can be converted into an alkyne in two steps. • These reactions are typically E2 reactions and occur via an alkenyl halide. • The strong base is needed in order to cause elimination of the alkenyl halide. What is the alkyne product from the reactions of the following with NaNH2 : (a) 2,2-dibromopropane (c) 1,2-dibromohexane (b) 1,1-dibromooctane (d) 2,3-dibromohexane .
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