Organic Chemistry CHE111 Lecture 6 & 7 Alkanes & Cycloalkanes PDF

Faculty of Science Organic Chemistry CHE111 Lecture 6 and 7: Alkanes & Cycloalkanes Prepared by: Ahmed Ragab 2024/2025 Organic compounds Alkanes (Paraffins) ❑ Alkanes...

Faculty of Science Organic Chemistry CHE111 Lecture 6 and 7: Alkanes & Cycloalkanes Prepared by: Ahmed Ragab 2024/2025 Organic compounds Alkanes (Paraffins) ❑ Alkanes are saturated hydrocarbons (i.e., contains only C-C single bonds) with a general formula of CnH2n+2 (n=1, 2, 3, ….) ❑ All carbon atoms in alkanes are sp3 hybridized, with a tetrahedral geometry (bond angle ~ 109.5°) Nomenclature of alkanes (IUPAC) ❑ The IUPAC name of an unbranched alkane consists of two parts: Stem (number of C-atoms) + Suffix (ane)  Nomenclature of alkanes (IUPAC) ❑ IUPAC nomenclature branched or substituted alkanes Prefix + parent alkane The position The longest and type of carbon chain substituents ❑ Substituent can be alkyl group, halogen atom, …etc ❑ Nomenclature of alkyl groups (R): ▪ An alkyl group is obtained by removal of one hydrogen atom an alkane. ▪ The name of alkyl group is derived from that of the corresponding alkane by replacing the suffix –ane by –yl. ❑ Halogen substituents:  Nomenclature of alkanes (IUPAC) ❑ IUPAC rules for nomenclature of branched or substituted alkanes: 1. Locate the longest continuous chain of carbon atoms, and use it as the parent name for the alkane Parent alkane: hexane Parent alkane: heptane 2. Number the longest chain beginning with the end nearer the substituent. Use the numbers to designate the location of the substituent group  Nomenclature of alkanes (IUPAC) ❑ IUPAC rules for nomenclature of branched or substituted alkanes: 3. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain The substituent groups should be listed alphabetically (i.e., ethyl before methyl) 4. When two substituents are present on the same carbon atom, use that number twice  Nomenclature of alkanes (IUPAC) ❑ IUPAC rules for nomenclature of branched or substituted alkanes: 5. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on 6. When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents:  Nomenclature of alkanes (IUPAC) ❑ IUPAC rules for nomenclature of branched or substituted alkanes: 7. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference  Quiz ❑ Provide an IUPAC name for the following alkane. ❑ Which structure does not represent 2-methylpentane? Preparation of alkanes Preparation of alkanes 2. From reduction of carbonyl compounds by Clemmensen reduction: CaO CaO NB: The alkane produced by this method has the double of carbon atoms of the reacted alkyl halide  Physical properties of alkanes ❑ The boiling points of the unbranched alkanes show a regular increase with increasing the carbon atoms. ❑ Branching of the alkane chain, however, lowers the boiling point. With branching the shape of the molecule tends to approach that of a sphere, and as this happens the surface area decreases, and the intermolecular forces (dispersion forces) become less effective.  Physical properties of alkanes ❑ Because they are non-polar, alkanes are insoluble in water but soluble in non-polar or slightly polar solvents such as other alkanes, diethyl ether or benzene.  Chemical properties of alkanes ❑ Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. Both of these bonds are non-polar. Therefore, there is no portion of the molecule that carries any significant amount of positive or negative charge, which is required for other ionic and polar molecules to be attracted to it. Therefore, alkanes generally do not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents. ❑ However, alkanes can undergo few reactions such as halogenation and combustion. Strong carbon-carbon single Both of these bonds are non-polar Strong carbon-hydrogen bonds no positive or negative charge  Chemical properties of alkanes 1. Halogenation ( free radical substitution reaction) Chloromethane dichloromethane trichloromethane tetrachloromethane Mechanism:  Chemical properties of alkanes 1. Halogenation propane 2-methylpropane Important Classification of carbon  Chemical properties of alkanes Note Important Classification of carbon Remember Stability of carbocation and Carbanion Cycloalkanes (Cn H2n) ❑ The general formula of cycloalkanes resembles that of alkenes (CnH2n). ❑ IUPAC nomenclature: 1. Cycloalkanes are named by placing the prefix cyclo- before the alkane name that corresponds to the number of carbon atoms in the ring. Cycloalkanes (Cn H2n) ❑ IUPAC nomenclature: 2. Monosubstituted cycloalkanes are named by placing the name of the substituent to the beginning of the parent cycloalkane name (No numbering). Methylcyclobutane Chlorocyclohexane 3. If there are several substituents, numbering is required. ▪ Give the least number to the first substituent in alphabetic 4 3 2 order. 1 ▪ Number the ring in such a way that gives the next 5 substituent the lowest possible number. 1-Ethyl-3-methyl- cyclopentane Cycloalkanes (Cn H2n) ❑ Preparation: 1) Dehalogenation of dihaloalkanes Br Zn + ZnBr2 Br 2) Reduction of cycloalkenes H Pd + H2 H Cycloalkanes (Cn H2n) ❑ Bond angle strain in cyclopropane and cyclobutane ❑ Cyclopentane has a small amount of ring strain, while cyclohexane is able to adopt the perfect geometry of a cycloalkane in which all angles are the ideal 109.5° Cycloalkanes (Cn H2n) ❑ Reactions: 1) Cyclopropane and cyclobutane undergo ring-opening reactions to relieve the ring strain and produce the more stable open-chain compounds. H2 / Ni Br2 CH3CH2CH3 Br CH2 CH2 CH2 Br 80 oC + H2 Ni CH3CH2CH2CH3 o 200 2) Cyclopentane and cyclohexane react differently. Ni + H2 No Reaction ▪ Cyclopentane does not react with H2 because the ring strain H Cl is very low + Cl2 h ▪ Cyclohexane has no ring strain and reacts with Cl2 like an + HCl open-chain alkane (substitution reaction)