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Terminal alkynes have the formula RC≡CH, where at least one end of the alkyne is a hydrogen atom. An example is methylacetylene (propyne using IUPAC nomenclature). They are often prepared by alkylation of monosodium acetylide. [4] Terminal alkynes, like acetylene itself, are mildly acidic, with pK a values of around 25.
For terminal alkynes it is sometimes important to mask the acidic hydrogen atom. This normally proceeds from deprotonation (via a strong base like methylmagnesium bromide or butyllithium in tetrahydrofuran/ dimethylsulfoxide ) and subsequently reaction with chlorotrimethylsilane to a terminally TMS-protected alkyne. [ 95 ]
A hydroboration reaction also takes place on alkynes. Again the mode of action is syn and secondary reaction products are aldehydes from terminal alkynes and ketones from internal alkynes. In order to prevent hydroboration across both the pi-bonds, a bulky borane like disiamyl (di-sec-iso-amyl) borane is used. [5]
In organic chemistry, alkynylation is an addition reaction in which a terminal alkyne (−C≡CH) is added to a carbonyl group (C=O) to form an α-alkynyl alcohol (R 2 C(−OH)−C≡C−R). [1] [2] When the acetylide is formed from acetylene (HC≡CH), the reaction gives an α-ethynyl alcohol. This process is often referred to as ethynylation.
It is a terminal alkyne. The compound is a common terminal alkyne substrate in diverse studies of catalysis. It is a colorless combustible gas. [1] 1-Butyne participates in reactions typical for terminal alkynes, such as alkyne metathesis, [2] hydrogenation, condensation with formaldehyde.
It is a terminal alkyne, in fact the smallest that is liquid at room temperature. The compound is a common terminal alkyne substrate in diverse studies of catalysis. The compound is a common terminal alkyne substrate in diverse studies of catalysis.
This is the most common method for preparing alkynyllithium reagents, because the terminal hydrogen bound to the sp carbon is very acidic and easily deprotonated. [36] For aromatic compounds, the position of lithiation is also determined by the directing effect of substituent groups. [ 63 ]
Usually two equivalents of sodium amide yields the desired alkyne. Three equivalents are necessary in the preparation of a terminal alkynes because the terminal CH of the resulting alkyne protonates an equivalent amount of base. Hydrogen chloride and ethanol can also be eliminated in this way, [11] as in the preparation of 1-ethoxy-1-butyne. [12]