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BU CHEM 102 - Saturated Hydrocarbons
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Chem 102 1st Edition Lecture 4Outline of Last Lecture I. SNOWDAY, but before that it was Chapter 12 : 1.7–1.12: Conformations of Alkanes, IUPAC Nomenclature of Alkanes, Line-Angle Structural Formulas for Alkanes, Classification of Carbon Atoms, Branched-Chain Alkyl Groups, CycloalkanesOutline of Current Lecture II. 1.13 – IUPAC Nomenclature for CycloalkanesIII. 1.14 – Isomerism of CycloalkanesIV. 1.15 – Physical properties of Alkanes and CycloalkanesV. 1.16 – Chemical Properties of Alkanes and CycloalkanesVI. 1.17 – Nomenclature and Properties of Halogenated Compounds Current Lecture1.13 - IUPAC Nomenclature for Cycloalkanes• Count the number of carbons in the ring, and name the parent cyclic chain (just like an alkane). • Add the prefix cyclo to the IUPAC name of the unbranched alkane, e.g., Cyclopentane• Name any alkyl groups on the ring in the usual way • Numbering conventions used in locating substituents on the ring: 1. If there is just one ring substituent, it is not necessary to locate it by number. 2. When two ring substituents are present, the carbon atoms in the ring are numbered beginning with the substituent of higher alphabetical priority and proceeding in the direction (clockwise or counterclockwise) that gives the other substituent the lower number. 3. When three or more ring substituents are present, ring numbering begins at the substituent that leads to the lowest set of location numbers. When two or more equivalent numbering sets exist, alphabetical priority among substituents determines the set used.1.14 - Isomerism of Cycloalkanes• Cycloalkanes also exhibit stereoisomerism. – Stereoisomers have the same molecular and structural formulas but different orientations of atoms in space. – Several forms of stereoisomerism exist. • Di-substituted Cycloalkanes exhibit Cis-Trans isomerism.– Cis–trans isomers have the same molecular and structural formulas but different spatial orientations of atoms due to the restricted rotation about bonds. • Cis- is a prefix that means “on the same side.” In cis1,2-dimethylcyclopentane, the two methyl groups are on the same side of the ring. • Trans- is a prefix that means “across from.” In trans-1,2dimethylcyclopentane, the two methyl groups are on opposite sides of the ring.1.15 - Physical properties of Alkanes and CycloalkanesSolubility and Density • Solubility: Follows the rule ‘like dissolves like’ – Soluble in non-polar solvents e.g., in hexanes – Insoluble in polar solvents e.g., water, alcohol – Application: protective wax coating on fruits to prevent loss of water as well as from fungi or bacterial strain etc. • Density: Lighter than water – Densities of alkanes and cycloalkanes range from – 0.6 – 0.8 g/mL (water density at 4 degree C = 1.0 g/mL) – Practical illustration: quick spreading of oil spills in water. – Fire caused by floating oil can not be extinguished by water.Boiling Points • The boiling points increase with an increase in the number of carbon atom in chain length or ring size. • Branching in carbon chain lowers the boiling point of an alkane. • Cycloalkanes have higher boiling points than their noncyclic counterparts with the same number of c-atoms Combustion • Vigorous reaction with oxygen (usually from air) • Accompanied by the evolution of heat and light • Always an exothermic process • The final products of any complete combustion of hydrocarbons are: – Carbon dioxide and Water (vapor) – CH4 + 2O2  CO2 + 2H2O + Energy – 2C6H14 + 19O2  12CO2 + 14H2O + Energy – Incomplete / partial combustion results in a mixture of carbon dioxide and carbon monoxide with water (g)1.16 - Chemical Properties of Alkanes and CycloalkanesHalogenation • Direct reaction with halogens (chlorine, bromine, iodine) • Substitution of one or more H-atoms of hydrocarbons by halogen atoms (substitution reaction of alkanes)1.17 - Nomenclature and Properties of Halogenated Compounds• Halogenated Alkanes: Derivatives alkanes in which one or more hydrogen atoms are substituted by halogen atoms. • Halogenated cycloalkanes: Cycloalkane derivatives in which one or more hydrogen atoms are substituted by halogen atoms • These derivatives are named as: – Haloalkanes or Halocycloalkanes (IUPAC naming) – Alkyl Halides or Cycloalkyl halides (common naming) IUPAC Rules for naming Halogenated Alkanes • Follow the same rules for naming alkanes / cycloalkanes • Lowest location number is given to the top priority group. • Alkyl and halogen substituents have the same ranking. • If 2 or more different substituents are present, the alphabetical order determines their priority.Physical Properties of Halogenated Alkanes • Density: Many halogen derivatives are heavier than water. • Polarity: They are mostly polar compounds (if unsymmetrical). • Boiling and Melting Points: – Generally higher than parent hydrocarbons. – Increase as the size of the alkyl increase – Increases as the size of the halogen atom


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