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Unformatted text preview: CHAPTER 10 ORGANIC CHEMISTRY (IB TOPICS 10 AND 20) SUMMARY Organic chemistry is the study of carbon compounds (with some exceptions). Carbon, electronic configuration 2.4 (1s22s22p2), is an element of Group IV/4, period 2and forms four covalent bonds by sharing electrons. C atoms combine with each other and with other nonmetal atoms and form bonds (C-C, C-H bonds) of high stability. This explains catenation, the occurrence of chains of carbon atoms to form a large number of stable compounds. Carbon also forms stable multiple bonds with itself, and with N and O atoms. Organic compounds can be classified into families of compounds called a homologous series with the same functional groups with similar chemistry. A functional group (such as C=C alkene, -OH in alcohols, -NH2 in amines, -CHO in aldehydes, -C=O carbonyl group in ketones) consists of an atom or group of atoms. Characteristics of any homologous series: • Molecular formula of any member differs from its predecessor by CH2 (methylene group). • Represented by a general formula: CnH2n+2 for alkanes (if n=3, then C3H8). • Similar chemical properties (due to the same functional group). • Gradual change in the physical properties e.g., the boiling increase gradually. Nomenclature The modern system for naming compounds is based on the names of the alkanes, and uses the IUPAC rules (although trivial/historical names are still often used). • Select the longest continuous carbon-chain and name it according to the parent alkane, alkene or alkyne. • Any group that is not a part of the longest continuous chain becomes a branch or substituent in the chain. The C atoms on the longest chain are numbered in order to identify the branches. The end of the chain from which the numbering starts is chosen so as to give the branches the lowest numbers • Branches are located by the number of the C atom on the chain • If two or more of the same branch are present, use di for two, tri for three etc. in the naming If several different branches are present, these are written in alphabetical order (ethyl before methyl), and the name is written as one word with commas between numbers and a dash between a number and the name, eg: 1,1-dichloro-2-iodoethane. Isomers: Isomers are different compounds with the same molecular formula (having different physical and/or chemical properties; at least one property must be different). Structural isomers: Same molecular formula but different structural formula. Chain isomers differ in the carbon chain, e.g.: butane and 2-methylpropane. Position isomers have the same functional group but in a different position on the carbon chain, e.g.: propan-1-ol and propan-2-ol. Functional group isomers contain different functional groups, e.g.: propanal and propanone. Stereoisomerism Geometric Isomerism: Requires (i) no rotation around at least one bond and (ii) 2 bulky groups. Gives rise to cis and trans isomers, e.g.: trans-1,2-dichloroethene (non-polar) and cis1,2-dichloroethene (polar). Similarly, trans-1,3-dichlorocyclobutane (non-polar) and cis-1,3dichlorocyclobutane (polar). Optical Isomers: Non-superimposable mirror images called enantiomers that rotate the plane of polarized light in opposite directions, i.e., differ in their optical activity, the ability to rotate the plane of polarized light. An asymmetric molecule (C atom with 4 different groups represented as C*) containing a chiral center has optical isomers. Living systems have a strong preference for one form of the optical isomers, e.g., L-aspartame is sweet (Nutrasweet) but the D-enantiomer is bitter. All amino acids, except aminoethanoic acid can show optical activity, e.g., 2-aminopropanoic acid. © IBID Press 2007 1 CHAPTER 10 ORGANIC CHEMISTRY (IB TOPICS 10 AND 20) SUMMARY • Plane polarized light is light vibrating in only one plane. • An optically active compound is one that is capable of rotating plane-polarized light. • An asymmetric molecule containing a chiral center can exist as two different isomers, called enantiomers, that are non-superimposable mirror images, and exhibit optical activity. • A carbon atom with four different groups rotates the plane of polarized light; thus such molecules are chiral (asymmetric), and the carbon atom is identified as C*. The molecule is asymmetric because of tetrahedral geometry around the carbon atom. • The rotation by a pair of enantiomers is equal but of opposite sign. • An equi-molar (meaning same concentration) mixture of a pair of enantiomers is optically inactive (does not rotate plane of polarised light) and is called a racemic mixture. • Physical properties of enantiomers are identical except rotation of plane polarized light. • Chemical properties of enantiomers are identical with non-optically active compounds. Combustion of Hydrocarbons: In the presence of oxygen, hydrocarbons are oxidized when the necessary activation energy, Ea is supplied; that is, when hydrocarbons are ignited, combustion takes place. In the presence of excess O2, the combustion tends to be complete, producing CO2 (g) and H2O (l) − combustion reactions are exothermic. CO2 is a greenhouse gas that absorbs infrared radiation reflected from the Earth’s surface and causes global warming. Incomplete combustion in the presence of O2 as a limiting reagent leads to the formation of poisonous carbon monoxide, CO as well as carbon, C (soot particles). CO is an odorless gas that readily reacts with the red pigment, hemoglobin, in the blood to form carboxyhemoglobin, a stable complex. Reactivity of alkenes: Alkenes are more reactive than alkanes. Alkenes contain a double bond consisting of a strong σ bond and a weaker π bond that is broken; this requires energy. However two strong σ bonds are formed in its place releasing energy; hence alkenes are energetically unstable with respect to their products in addition reactions (or products are more stable). A double bond has a much higher e− density (serves as a source of electrons), attracting e− deficient groups called electrophiles in addition reactions. The excess e− density in alkenes (and alkynes) implies nucleophilic behavior. Typical Reactions of Alkenes: Addition Characterized by addition reactions in which the C=C double bond is converted into a single bond and atoms (or groups of atoms) are added to each of the 2 carbon atoms across the double bond. CH2=CH2 + X—Y → CH2X—CH2Y. 1. With H2 (g) passed over nickel as catalyst at 150°C: Hydrogenation: Ni ( );150 C CH2=CH2 + H2 ⎯⎯s⎯⎯→ CH3-CH3 (C2H6: ethane produced) 2. With Cl2 and Br2, reaction can take place in the dark; no u.v. light, heat or catalyst is required: CH2=CH2 + Br2 → CH2Br—CH2Br; 1,2-dibromoethane (not 1,1-dibromoethane) © IBID Press 2007 2 CHAPTER 10 ORGANIC CHEMISTRY (IB TOPICS 10 AND 20) SUMMARY SN2 mechanism is for primary halides – a one step mechanism: nucleophile approaches from side opposite to X (inversion of configuration takes place); two species involved in slow step ∴bimolecular. The predominant mechanism for tertiary halogenoalkanes is SN1 and for primary halogenoalkanes is SN2. Both mechanisms occur for secondary halogenoalkanes. Regardless of the mechanism, the overall change in all nucleophilic substitution reactions is that the nucleophile, the attacking group reacts with the alkyl halide, R–X and substitutes for the halide ion, X−, the leaving group: Nu− + R–X R–Nu + X−. Regardless of the mechanism, the overall change in all nucleophilic substitution reactions is that the nucleophile, the attacking group reacts with the alkyl halide, R–X and substitutes for the halide ion, X, the leaving group: Nu + R–X R–Nu + X. Effect of Attacking Nucleophiles and Different Leaving Halides on Nucleophilic Substitution Mechanism: 1. The Nucleophile: For SN1 reactions, rates depend only on the concentration of the tertiary halide, and not on the nucleophile (unimolecular). Thus changing the nucleophilic agent will not change the rate of an SN1 reaction as it is not involved in the slow, rate determining step. However, for an SN2 mechanism where rate = k [R– X]1[Nu–]1, the more strongly nucleophilic the reagent, the faster the rate of reaction. Thus, the strong base OH– is a more strongly nucleophilic agent than H2O (polarity difference) and undergoes a faster rate of substitution in the SN2 mechanism. 2. The Halogen: In both SN1 and SN2 mechanisms, the weaker the C-halide (C-X) bond, the faster the reaction, since the leaving group will break off more easily. Thus for the C-X bonds, the rate of reaction is: C–I > C–Br > C–Cl > C–F, because as the size increases down the halogen group, the bonding electrons in C–I are the furthest apart. It is therefore the longest and weakest bond whereas C–F would be the shortest and strongest bond. Thus the halide leaves the fastest in the order I– > Br– > Cl– > F– and the rate of reactivity decreases from alkyl iodide to alkyl fluoride. 3. Nature of the halogenoalkane Influences Rate of Reactivity: (i) Tertiary > secondary > primary. This is because a tertiary carbocation is more stable than a secondary which is more stable than a primary carbocation. (ii) SN1 > SN2: This is because SN1 mechanism involves tertiary halides and the spontaneous formation of the tertiary carbocation, whereas SN2 mechanism involves a transition state of relatively high activation energy, Ea. N.B. Aromatic halides are much less reactive than the halogenoalkanes. The lone electron pair on the halogen atom delocalizes with the π electron cloud of the benzene ring. Thus the C–X has greater electron density, is stronger and more difficult to break. Also, the electron rich aromatic ring repels a nucleophile (also electron rich). (N.B. Shading indicates Ch 20 (AHL) material.) © IBID Press 2007 3 ...
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