Alkyne Lecture 1 PDF

Alkynes Lecture 1 Organic chemistry Introduction Alkanes are saturated hydrocarbons; meaning a single bond between the carbon atoms; alkenes are unsaturated hydrocarbons meaning one or more double bonds between carbon atoms; alkynes are also unsaturated hydrocarbons with one or more triple bonds bet...

Alkynes Lecture 1 Organic chemistry Introduction Alkanes are saturated hydrocarbons; meaning a single bond between the carbon atoms; alkenes are unsaturated hydrocarbons meaning one or more double bonds between carbon atoms; alkynes are also unsaturated hydrocarbons with one or more triple bonds between the carbon atoms. Introduction Introduction Alkyne Structure Alkynes contain a carbon-carbon triple bond. An alkyne has the general molecular formula CnH2n−2, giving it four fewer hydrogens than the maximum possible for the number of carbons present. The triple bond introduces two degrees of unsaturation. Alkyne Structure Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond. Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond. Alkyne Bonding Recall that the triple bond consists of 2 π bonds and 1 σ bond. Each carbon is sp hybridized with a linear geometry and bond angles of 180o. Strength of Alkyne Bonds Bond dissociation energies of the C − C bonds in ethylene (one σ and one π bond) and acetylene (one σ and two π bonds) can be used to estimate the strength of the second π bond of the triple bond. Nomenclature Alkynes are named in the same general way that alkenes are named. In the IUPAC system, change the –ane ending of the parent alkane name to the suffix –yne. Choose the longest continuous chain that contains both atoms of the triple bond and number the chain to give the triple bond the lower number. Example: Give the IUPAC name of the following alkyne: Nomenclature The simplest alkyne, H-C≡C-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name. Nomenclature The two-carbon alkyl group derived from acetylene is called an ethynyl group; Compounds with two triple bonds are named as diynes, those with three are named as triynes and so forth. Nomenclature Compounds with both a double and triple bond are named as enynes. The chain is numbered to give the first site of unsaturation (either C=C or C≡C) the lower number. Priority of Functional Groups Suffixes Physical Properties of Alkynes The physical properties of alkynes resemble those of hydrocarbons of similar shape and molecular weight: 1. Alkynes have low melting points and boiling points. 2. Melting point and boiling point increase as the number of carbons increases. 3. Alkynes are soluble in organic solvents and insoluble in water. Acetylene The simplest alkyne, H−C≡C−H, named in the IUPAC system as ethyne, is more often called acetylene, its common name. Acetylene (H−C≡C−H) is a colorless gas with an ethereal odor that burns in oxygen to form CO2 and H2O. The combustion of acetylene releases more energy per mole of product formed than any other hydrocarbons. When combined with oxygen, it burns with a very hot flame and is used in welding. Preparation of Alkynes Alkynes are prepared by elimination reactions. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne through two successive E2 elimination reactions. Note: geminal dihalide (both halogens on the same carbon) vicinal dihalide (halogens on adjacent carbons) Preparation of Alkynes Example: Preparation of Alkynes from Alkenes Convert alkene A into alkyne B by stepwise method. Since vicinal dihalides are readily made from alkenes, one can convert an alkene to the corresponding alkyne in a two-step process involving: 1. Addition of X2 ( halogenation) forms vicinal dihalide. 2. Elimination of two equivalents of HX forms two π bonds. Introduction to Alkyne Reactions All reactions of alkynes occur because they contain easily broken π bonds or, in the case of terminal alkynes, an acidic, sp hybridized C‒ H bond. 1. Addition Reactions of Alkynes Like alkenes, alkynes undergo addition reactions because they contain relatively weak π bonds. Two sequential reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds. 2. Terminal Alkynes- Reaction as an acid Sp hybridized C − H bonds are considerably more acidic than sp2 and sp3 hybridized C − H bonds. The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus. 2. Terminal Alkynes- Reaction as an acid 2. Terminal Alkynes- Reaction as an acid Formation of Acetylide Ions Therefore, terminal alkynes are readily deprotonated with strong base in a Brønsted-Lowry acid-base reaction. The resulting ion is called the acetylide ion. Formation of Acetylide Ions Four Addition Reactions of Alkynes Alkynes react with electrophiles. Four addition reactions are illustrated in this figure using 1- butyne as starting material. Hydrohalogenation Electrophilic Addition of HX Alkynes undergo hydrohalogenation, i.e., the addition of hydrogen halides, HX (X = Cl, Br, I). Two equivalents of HX are usually used: addition of one mole forms a vinyl halide, which then reacts with a second mole of HX to form a geminal dihalide. Hydrohalogenation – Markovnikov’s Rule Hydrohalogenation Mechanism Hydrohalogenation Mechanism Hydrohalogenation Mechanism