Organic Chemistry is the chemistry of compounds containing.

Transcription

1 Chapter 21 Lecture Notes Organic Chemistry Intro Organic Chemistry is the chemistry of compounds containing. The Bonding of Carbon Because carbon has four valence electrons, it can form covalent bonds. Carbon forms singe, double, and triple bonds to achieve a filled octet. A unique ability of carbon: it can with other carbons to form Hydrocarbons They contain only carbon and hydrogen. There are three main groups of hydrocarbons: hydrocarbons: only single bonds between carbon atoms. hydrocarbons: contain double or triple bonds between carbon atoms. hydrocarbons: contain a benzene ring. Saturated Hydrocarbons Alkanes Alkanes are acyclic, saturated hydrocarbons that form a series of compounds with the general formula The simplest alkane is methane: CH

2 Name Molecular Structural Formula Formula Condensed Formula BP Phase/ use CH C CH 3 CH C CH 3 CH 2 CH C CH 3 CH 2 CH 2 CH 3 aka CH 3 (CH 2 ) 2 CH C CH 3 (CH 2 ) 3 CH 3 36 C CH 3 (CH 2 ) 4 CH C CH 3 (CH 2 ) 5 CH C CH 3 (CH 2 ) 6 CH C CH 3 (CH 2 ) 7 CH 3 CH 3 (CH 2 ) 8 CH 3 Increased boiling point with increasing size due to increases in: 21-2

3 Shorthand (Line-Bond) Notation for Aliphatic Hydrocarbons Writing out even the condensed formula for organic molecules can become very burdensome, because of the large size of many molecules. Each bond is represented by a line. The end of each line is shown assumed to be a carbon atom, unless expressly shown. Each carbon atom must have 4 bonds. Any bonds not shown are assumed to be C-H bonds. (C atoms will not have lone pairs of electrons.) Halogens, O, S, and N atoms require a full octet. If they have less than a full octet in the linebond structure, they fill the octet with lone pairs of electrons. Simple examples: Propane C3H8 Butane C4H10 Octane C8H18 Remember to count the initial carbon! Constitutional Isomers and Branched-Chain Alkanes In addition to straight-chain alkanes, branched-chain alkanes are possible. For example: Isobutane (2-methylpropane) has the structure: or or or structural formula condensed formula line-bond molecular Note that isobutane, C 4 H 10 has the same molecular formula as normal butane. Normal and isobutene are constitutional (or structural) isomers: compounds with the same molecular formula but different structural formulas. 21-3

4 The following are NOT isomers, they are just different ways to draw the same compound: LP #1. Write condensed formulas and skeletal line-bond structures for the following structural formulas: 21-4

5 LP #2.Draw the line-bond structure for: CH 3 CH 2 CHCH 2 CHCHCH 3 OH Naming Hydrocarbons We will only cover naming alkanes and substituted alkanes. The names of the segments derived from the basic hydrocarbons are as follows: Plus the remainder of the the alkanes up to C10. Naming Rules The following rules are applied to naming alkanes: 1. Determine the longest continuous (not necessarily straight) chain of carbon atoms to determine the base name. 2. Any chain branching off the longest chain is named as an alkyl group and is identified prior to the base name. 3. Use a number that locates the branch s position on the longest chain and place it before the name of the alkyl group. (Start numbering from the end with a branch closest to it.) 4. When there is more than one branch of the same kind, use a Greek prefix (di, tri, etc.) to specify how many there are and include a location number for each. 5. When there are two or more different branches, list them in alphabetical order. 21-5

6 LP#3. Name the following hydrocarbons: Cycloalkanes Cycloalkanes are cyclic, saturated hydrocarbons that form another homologous series, in which the carbon atoms are joined in a Below are the structures of some simple cycloalkanes: 21-6

7 Reactions of Alkanes Reactions of Alkanes with Oxygen All hydrocarbons burn in excess oxygen to produce carbon dioxide, water, and heat. Combustion: For example: Substitution Reactions of Alkanes A substitution reaction is one in which a part of the reacting molecule is substituted for an H atom on a hydrocarbon. For example, the reaction between ethane and chlorine: The product of the first reaction may undergo further substitutions so that you end up with a product that is a Unsaturated Hydrocarbons Alkenes and Alkynes The alkenes and alkynes are unsaturated (cyclic or acyclic) that contain carbon-carbon double or triple bonds. Alkenes have Alkynes have Under proper conditions, molecular hydrogen can be added to an alkene or an alkyne to produce a saturated compound in a process called catalytic hydrogenation. 21-7

8 Alkenes and Geometric Isomers Alkenes are hydrocarbons with the general formula. The simplest alkene is ethene (a.k.a. ethylene) : Alkenes contain a carbon-carbon. Also known as olefins. They lie in a flat plane around the double bond. Double bonds This leads to geometric isomers. Stereoisomers Geometric isomers: isomers in which some atoms occupy different relative positions in space. For example: 2-butene has two geometric isomers: The isomers are given prefixes: means that the substituted carbons are on the same side of the double bond. means that the substituted carbons are on opposite sides of the double bond. Predicting Cis-Trans Isomerism It is not possible to have cis-trans isomers if both groups attached to the same side of a double bonded carbon are identical. Cis vs Trans Fats 21-8

9 LP#4. For which of the following compounds can cis-trans isomers exist? If they are possible, draw them. 1. CH3CH=CHCH2CH3? 2. CH3CH2C=CHCH3? Optical Isomers CH2CH3 Compounds which have the same sequence of bonding connections, but which are not superimposeable on themselves. They are called chiral molecules. These usually include a carbon with 4 uniquely different attachments. They rotate plane polarized light. Reactions of Alkenes Addition Reactions of Alkenes Alkenes are more reactive than alkanes because many reactants add to the double bond. An addition reaction is a reaction in which parts of a reactant are added to each carbon atom of a carbon-carbon double bond, which converts to a carbon-carbon single bond. A simple example of an addition reaction is the addition of a halogen such as bromine to the double bond in propene. CH3CH=CH2 + Br2 This can be used as a test for 21-9

10 Unsymmetrical reagents such as HCl add to give 2 different products that are constitutional isomers. However, the two products are not made in equal amounts. Markownikoff s ( aka Markovnikov s) Rule: a generalization that states that the major product of an addition reaction by an unsymmetrical reagent such as HCl is the one obtained when the H in the reagent adds to the side of the double bond that already has the greater number of H atoms. This would predict a predominant amount of the product shown. LP#5. Predict the major product of the following addition reaction. Cl CH2==CH + HCl Alkynes Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond. The general formula is. The simplest alkyne is ethyne (a.k.a. acetylene) Alkynes undergo addition reactions in the same manner as alkynes, simply adding 2 molecules or reagent at the triple bond. Markovnikov s Rule is still followed for each addition step. LP#6. Predict the major product of the following addition reaction. CH3 C C H + 2HCl 21-10

11 Aromatic Hydrocarbons Aromatic hydrocarbons contain six-membered rings of carbon atoms with alternating single and double carbon-carbon bonds. The ring is sometimes shown with a circle in the center instead of the alternating lines. The term aromatic implies that compounds that contain a benzene ring have and this is true for many of them. These compounds are all around us. For example Cinnamaldehyde is responsible for the aroma of Methyl salicylate is responsible for the aroma of 21-11

12 Aromatic hydrocarbons are also the basis of many drugs, both legal and illegal. LP#7. Determine the molecular formula of each of the above compounds. The molecular formula should be listed in the order of C, H, N, O, S, halogen. Substitution Reactions of Aromatic Hydrocarbons Although benzene rings appear unsaturated, the double bonds DO NOT undergo addition reactions because they are so stable. A test for unsaturation will be negative! Instead, they typically undergo substitution reactions: where one attachment on the benzene ring is substituted with another. For example, Br can be substituted for H by: NO2 can be substituted for H by: 21-12