Alkynes
Chapter Name : Hydrocarbons |
Sub Topic Code : 102_11_13_04_07 |
Topic Name : Alkynes |
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Sub Topic Name : Acidic Character Of Alkyne |
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Introduction
In organic chemistry, an alkyne is an unsaturated hydrocarbon which has at least one carbon—carbon triple bond between two carbon atoms. The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature.
Pre-Requisites:
Alkynes
Activity:
Plastics.
Real Life Question:
What are alkynes?
Key Words / FlashCards
| Key Words | Definitions (pref. in our own words) |
|---|---|
| Bronsted acids | Bronsted–Lowry theory is an acid–base reaction theory, proposed independently by Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory is that an acid (or Bronsted acid) is defined as being able to lose, or "donate" a proton (the hydrogen cation, or H+) while a base (or Bronsted base) is defined as a species with the ability to gain, or "accept," a proton. |
Learning aids / Gadgets
| Gadgets | How it can be used |
|---|---|
| Alkynes | Alkynes are generally nonpolar molecules with little solubility in polar solvents, such as water. |
Real life uses :
Alkanes are our fuels. From the gases, methane, to butane, The liquid fuel for cars, in theory about C8 alkanes, Kerosene for heating and aircraft, All the way up to heavy fuel oils.
Places to visit :
Chemistry laboratory.
Practical examples around us
| Examples | Explainations |
|---|---|
| ethyne | the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities. |
What you learn in Theory:
Alkanes are undoubtedly the weakest Brönsted acids commonly encountered in organic chemistry. It is difficult to measure such weak acids, but estimates put the pKa of ethane at about 48. Hybridizing the carbon so as to increase the s-character of the C-H increases the acidity, with the greatest change occurring for the sp-C-H groups found in terminal alkynes. Thus, the pKa of ethene is estimated at 44, and the pKa of ethyne (acetylene) is found to be 25, making it 1023 times stronger an acid than ethane. This increase in acidity permits the isolation of insoluble silver and copper salts of such compounds.
What you learn in Practice:
Despite the dramatic increase in acidity of terminal alkynes relative to other hydrocarbons, they are still very weak acids, especially when compared with water, which is roughly a billion times more acidic. If we wish to prepare nucleophilic salts of terminal alkynes for use in synthesis, it will therefore be necessary to use a much stronger base than hydroxide (or ethoxide) anion. Such a base is sodium amide (NaNH2), discussed above, and its reactions with terminal alkynes may be conducted in liquid ammonia or ether as solvents. The products of this acid-base reaction are ammonia and a sodium acetylide salt. Because the acetylide anion is a powerful nucleophile it may displace halide ions from 1º-alkyl halides to give a more highly substituted alkyne as a product (SN2 reaction). This synthesis application is described in the following equations. The first two equations show how acetylene can be converted to propyne; the last two equations present a synthesis of 2-pentyne from propyne.