Lecture 6 Org1 New PDF - Unsaturated Hydrocarbons

Unsaturated Hydrocarbons Alkynes Alkynes are nonpolar molecules that are held together through London forces of attraction. Increasing molecules size leads to stronger London force attractions and higher melting and boiling points...

Unsaturated Hydrocarbons Alkynes Alkynes are nonpolar molecules that are held together through London forces of attraction. Increasing molecules size leads to stronger London force attractions and higher melting and boiling points 1  Nomenclature IUPAC rules: 1. The parent chain is the longest chain of carbon atoms that contains the carbon-carbon triple. 2. Numbering the parent begins at the end nearer to the triple bond. Alkynes end with “yne” 3. The position of the multiple bond is written directly in front of the hydrocarbon parent chain name. 4. Substituents are numbered as in alkanes 2  Nomenclature IUPAC rules: CH3 C C CH2 CH2 Br 5-bromo-2-pentyne CH3 CH3 CH3 CH CH2 C C CH CH3 2,6-dimethyl-3-heptyne Question: Name the following compound 3  Nomenclature Alkenes and alkynes are considered to have equal priority If both double and triple bond are in the same molecule, any of them that is closer to the end of the chain determines the direction of numbering. When both double and triple bonds have the same position number, the double bond takes the lower number. In the name, “ene” comes before “yne” because of alphabetical order. 4 Nomenclature 5 Nomenclature 6  Physical properties Alkynes have physical properties similar to those of corresponding alkanes and alkenes. Alkynes up to four carbons are gases at room temperature. Being relatively nonpolar themselves, alkynes dissolve in nonpolar solvents or in solvents of low polarity. 7  The acidity of terminal alkynes An alkyne hydrogen is weakly acid. Alkene and alkane hydrogens are not acidic. This is explained on the basis of different hybridization state of carbon atom in each of them. Electrons of 2s orbitals have lower energy than those of 2p orbitals. With hybrid orbitals, having more s character means that the electrons of the anion will, on the average, be lower in energy, and the anion will be more stable. 8  The acidity of terminal alkynes The sp orbitals of the C-H bonds of ethylene have 50% s character (one s orbital and one p orbital), those of the sp2 orbitals of ethene have 33.3% s character, while those of the sp3 orbitals of ethane have only 25% s character. An sp carbon atom of ethylene is more electronegative than the sp2 carbon atoms of ethene, which in turn is more electronegative than sp3 carbon atoms of ethane. Order of acidity of the Hydrocarbons: HCΞCH > H2C=CH2 > H3C-CH3 9  The acidity of terminal alkynes Monosubstituted acetylenes or 1-alkynes are called terminal alkynes, and the hydrogen attached to the carbon of the triple bond is called the acetylenic hydrogen atom 10  Preparation of alkynes Alkynes are prepared from alkyl dihalides via elimination of atoms or groups from adjacent carbons. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne  Carbon Carbon bond formation Terminal alkynes can be converted to a nucleophiles The acetylenic proton of ethyne or any terminal alkyne can be removed with a strong base such as sodium amide (NaNH2) giving an alkynide anion. Alkynide anion acts as a nucleophile and displaces the halide ion from the primary alkyl halides giving a longer chain alkyne. 12  Synthesis Question Outline a synthetic strategy of 4-phenyl-2-butyne from 1-propyne. 13  Reactions 1- Hydrogenation One or two molar equivalents of hydrogen will add to a carbon–carbon triple bond Reactions of alkynes: 1. Hydrogenation of alkynes: Addition of hydrogen to a triple bond can generate the alkane Ni CH3CH2CHCH2C CH + 2H2 CH3CH2CHCH2CH2CH3 CH3 CH3 H R' Na in NH3 R C C R' + H2 C C R Li tal H nd ys ca la t Trans-isomer r R R' HC C H Cis-isomer  Reactions 2- Addition of Br2 or Cl2 The addition may occur once or twice, depending on the number of molar equivalents of halogen  Reactions 3- Addition of hydrogen halides Alkynes react with one molar equivalent of HCl or HBr to form haloalkenes, and with two molar equivalents to form geminal dihalides. Both additions are regioselective and follow Markovnikov’s rule.  Reactions 3- Addition of hydrogen halides Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture.  Reactions 4- Addition of water Markovnikov addition of water in the presence of acid gives ketones  4. Acidic catalytic hydration of alkynes: Additions of water occur only once to the alkyne, diols do not form. Upon the addition of the first –OH, an enol forms. This enol exists in equilibrium with a keto (carbonyl) form. This equilibrium, called a tautomerization CH3CH2CH2C CCH2CH2CH3 + H2O CH3CH2CH2CH CCH2CH2CH3 enol-form OH O CH3CH2CH2CH2 CCH2CH2CH3 keto-form  Reactions 5- Oxidative cleavage of alkynes Ozonolysis of Alkynes Treating alkynes with ozone followed by acetic acid leads to cleavage at the carbon–carbon triple bond. The products are carboxylic acids.