Alkenes and Alkynes - CHEM111 Week 9 Lecture Notes PDF

ALKENES AND ALKYNES OUR LADY OF FATIMA UNIVERSITY – ANTIPOLO CAMPUS COLLEGE OF MEDICAL LABORATORY SCIENCE GENERAL, INORGANIC AND ORGANIC CHEMISTRY Three classes of unsaturated hydrocarbons: (on the basis of the type of multiple bond/s present) 1. Alkenes (one or more carbon-carbon double bonds) 2....

ALKENES AND ALKYNES OUR LADY OF FATIMA UNIVERSITY – ANTIPOLO CAMPUS COLLEGE OF MEDICAL LABORATORY SCIENCE GENERAL, INORGANIC AND ORGANIC CHEMISTRY Three classes of unsaturated hydrocarbons: (on the basis of the type of multiple bond/s present) 1. Alkenes (one or more carbon-carbon double bonds) 2. Alkynes (one or more carbon-carbon triple bonds) 3. Aromatic Hydrocarbons A special type of “delocalized” bonding that involves a six membered ring Functional Group is the part of an organic molecule where most of its chemical reactions occur carbon–carbon multiple bonds are the functional group for an unsaturated hydrocarbon ALKENES Alkenes - Is an acyclic unsaturated hydrocarbon that contains one or more carbon– carbon double bonds General molecular formula for alkene with one double bond: CnH2n, is same as cycloalkanes Functional group: C = C group The -ene ending means a double bond is present  Alkenes with more than one carbon–carbon double bond are relatively common. When two double bonds are present, the compounds are often called dienes; for three double bonds the designation trienes is used. CYCLOALKENE Is a cyclic unsaturated hydrocarbon that contains one or more carbon– carbon double bonds within the ring system General molecular formula of cycloalkenes with one double bond: CnH2n-2 ALKYNE is an acyclic unsaturated hydrocarbon that contains one or more carbon–carbon triple bonds General molecular formula with one triple bond: CnH2n- 2 The –yne ending means a triple bond is present IUPAC NOMENCLATURE FOR ALKENES AND ALKYNES: 1. Replace the alkane suffix -ane with the suffix –ene or - ynewhich is used to indicate the presence of a carbon– carbon double bond or triple bond 2. Select as the parent carbon chain the longest continuous chain of carbon atoms that contains both carbon atoms of the double bond or triple bond. For example, select 3. Number the parent carbon chain beginning at the end nearest the multiple bond so that the carbon atoms in that bond have the lowest possible numbers If the multiple bond is equidistant from both ends of the parent chain, begin numbering from the end closer to a substituent. 4. Indicate the position of the multiple bond using the lower numbered carbon atom of the bond. 5. If more than one multiple bond is present, number the chain from the end near the first multiple bond. If a double and triple bond are equidistant from the end of the chain, the double bond receives the lowest numbers. 6. If a molecule contains both a double and a triple bond, the carbon chain is numbered so that the first multiple bond gets a lower number. If both bonds can be assigned the same number, the double bond takes precedence. The molecule is then named "n-ene-n-yne", with the double bond root name preceding the triple bond root name 7. Use the suffixes -diene, -triene, -tetrene, and so on when more than one multiple bond is present in the molecule. A separate number must be used to locate each multiple bond. 8. Do not use a number to locate the double bond in unsubstituted cycloalkenes with only one double bond because that bond is assumed to be between carbons 1 and 2. 9. In substituted cycloalkenes with only one double bond, the double-bonded carbon atoms are numbered 1 and 2 in the direction (clockwise or counterclockwise) that gives the first- encountered substituent the lower number. Again, no number is used in the name to locate the double bond. 8. In cycloalkenes with more than one double bond within the ring, assign one double bond the numbers 1 and 2 and the other double bonds the lowest numbers possible.  3-Ethyl-4- 5-Methyl-2- methylcyclohe hexene xene 1,3-Butadiene 5-Methyl-1,3- cyclopentadiene PRACTICE Pent-1-ene-4- yne Hex-1-en-4- yne LINE-ANGLE STRUCTURAL FORMULAS FOR ALKENES CONSTITUTIONAL ISOMERISM IN ALKENES In general, there are more alkene isomers for a given number of carbon atoms than there are alkane isomers. This is because there is more than one location where a double bond can be placed in systems containing four or more carbon atoms.  ALKENE ISOMERISM ….. REACTIONS OF ALKANES, ALKENES AND ALKYNES REACTIONS OF ALKANES Combustion reaction – is a chemical reaction between a substance and oxygen (usually from air) that proceeds with the evolution of heat and light (usually as a flame) REACTIONS OF ALKANES Halogenation reaction – is a chemical reaction between a substance and a halogen in which one or more halogen atoms are incorporated into molecules of the substance An example of substitution reaction Substitution reaction – is a chemical reaction in which part of a small reacting molecule replaces an atom or a group of atoms on a hydrocarbon or hydrocarbon derivative Methane and chlorine, when heated to a high temperature or in the presence of light, react as follows: REACTIONS OF ALKENES  Combustion reaction Alkenes, like alkanes, are very flammable. The combustion products, as with any hydrocarbon, are carbon dioxide and water.  Addition reaction – is a reaction in which atoms or groups of atoms are added to each carbon atom of a carbon–carbon multiple bond in a hydrocarbon or hydrocarbon derivative A general equation for an alkene addition reaction is: Addition reactions can be classified as: 1. Symmetrical Addition Reaction is an addition reaction in which identical atoms (or groups of atoms) are added to each carbon of a carbon–carbon multiple bond 2. Unsymmetrical Addition Reaction is an addition reaction in which different atoms (or SYMMETRICAL ADDITION REACTION Two most common examples: 1. Hydrogenation 2. Halogenation Hydrogenation reaction - is an addition reaction in which H2 is incorporated into molecules of an organic compound Halogenation reaction - is an addition reaction in which a halogen is incorporated into molecules of an organic compound UNSYMMETRICAL ADDITION REACTION Two important types: 1. Hydrohalogenation 2. Hydration Hydrohalogenation reaction – is an addition reaction in which a hydrogen halide (HCl, HBr, or HI) is incorporated into molecules of an organic compound o In alkene hydrohalogenation one carbon atom of a double bond receives a halogen atom and the other carbon atom receives a hydrogen atom. Hydrohalogenation reactions require no catalyst. UNSYMMETRICAL ADDITION REACTION Hydration reaction – an addition reaction in which H2O is incorporated into molecules of an organic compound. One carbon atom of a double bond receives a hydrogen atom and the other carbon receives an –OH group. o In alkene hydrohalogenation one carbon atom of a double bond receives a halogen atom and the other carbon atom receives a hydrogen atom. Hydrohalogenation reactions require no catalyst.