Organic Chemistry Lecture Notes PDF 2021-2022

Chapter2 12 Chapter Lecture Lecture Fundamentals of General, Organic, and Biological Chemistry 8th...

Chapter2 12 Chapter Lecture Lecture Fundamentals of General, Organic, and Biological Chemistry 8th Edition McMurry, Ballantine, Hoeger, Peterson Chapter Twelve Introduction to Organic Chemistry: Alkanes Christina A. Johnson University of California, San Diego © 2017 Pearson Education, Inc.  Outline 12.1 The Nature of Organic Molecules 12.2 Families of Organic Molecules: Functional Groups 12.3 The Structure of Organic Molecules: Alkanes and Their Isomers 12.4 Drawing Organic Structures 12.5 The Shapes of Organic Molecules 12.6 Naming Alkanes 12.7 Properties of Alkanes 12.8 Reactions of Alkanes 12.9 Cycloalkanes 12.10 Drawing and Naming Cycloalkanes © 2017 Pearson Education, Inc.  12.1 The Nature of Organic Molecules Organic chemistry: The study of carbon compounds Identify the general structural characteristics of organic molecules, in particular, the tetravalent nature of carbon and the different ways in which it can be expressed. -Carbon is tetravalent( always forms four bonds). -Organic molecules have covalent bonds. -Carbon forms multiple covalent bonds by sharing more than two electrons with a neighboring atom. -Organic molecules have specific three- dimensional shapes. 1. A carbon that has 4 groups attached will be tetrahedral. 2. A carbon that has 3 groups attached will be trigonal planar. 3. A carbon that has 2 groups attached will be linear. © 2017 Pearson Education, Inc.  12.1 The Nature of Organic Molecules Organic molecules often contain nitrogen and oxygen in addition to carbon and hydrogen. Most organic compounds are insoluble in water. Almost all of those that are soluble do not conduct electricity. Only small polar organic molecules or large molecules with many polar groups dissolve in water. When carbon bonds to a more electronegative element, polar covalent bonds result. © 2017 Pearson Education, Inc.  12.2 Families of Organic Molecules: Functional Groups Learning Objectives: Define functional group. Identify the functional groups in organic molecules. Functional group: An atom or group of atoms within a molecule that has a characteristic physical and chemical behavior Organic compounds can be classified into families according to functional groups (structural features). A given functional group tends to undergo the same types of reactions in every molecule that contains it. The chemistry of an organic molecule is primarily determined by the functional groups it contains, not by its size or complexity. © 2017 Pearson Education, Inc.  12.2 Families of Organic Molecules: Functional Groups © 2017 Pearson Education, Inc.  12.2 Families of Organic Molecules: Functional Groups © 2017 Pearson Education, Inc.  Worked Example 12.1 To which family of organic compounds do the following compounds belong? Explain. Aromatics Alkenes Alcohols Amine Aromatic Ketones Amides Disulfide © 2017 Pearson Education, Inc.  Worked Example 12.2 Given the family of organic compounds to which the compound belongs, propose structures for compounds having the following chemical formulas: (a) An amine having the formula C2H7N NH2 (b) An alkyne having the formula C3H4 (c) An ether having the formula C4H10O O © 2017 Pearson Education, Inc.  12.3 The Structure of Organic Molecules: Alkanes and Their Isomers Learning Objective: Recognize structural (constitutional) isomers and functional group isomers. Alkane: A hydrocarbon that has only single bonds The general rule for all hydrocarbons except methane is that each carbon must be bonded to at least one other carbon. The carbon atoms bond together to form the “backbone” of the compound, with the hydrogens on the periphery. The general formula for alkanes is CnH2n+2 where n is the number of carbons in the compound. © 2017 Pearson Education, Inc.  12.3 The Structure of Organic Molecules: Alkanes and Their Isomers As larger numbers of carbons and hydrogens combine, the ability to form isomers arises. Compounds that have the same molecular formula but different structural formulas are called isomers. There are two ways in which molecules that have the formula C4H10 can be formed. © 2017 Pearson Education, Inc.  12.3 The Structure of Organic Molecules: Alkanes and Their Isomers A straight-chain alkane is an alkane that has all its carbons connected in a row. A branched-chain alkane is an alkane that has a branching connection of carbons. Constitutional isomers are compounds with the same molecular formula, but with different connections among their atoms. Also known as structural isomers. Constitutional isomers of a given molecular formula are: - chemically distinct from one another. - They have different structures and physical properties. © 2017 Pearson Education, Inc.  12.3 The Structure of Organic Molecules: Alkanes and Their Isomers When the molecular formula contains atoms other than carbon and hydrogen, the constitutional isomers obtained can also be functional group isomers. These are isomers that differ in both molecular connection and family classification. © 2017 Pearson Education, Inc.  Worked Example 12.3 Draw all isomers that have the formula C6H14. Solution: © 2017 Pearson Education, Inc.  12.4 Drawing Organic Structures Learning Objectives: -Draw structural, condensed, and line formulas for chemical compounds -Convert any one of them into its corresponding alternative. Condensed structure: A shorthand way of drawing structures in which C–C and C–H bonds are understood rather than shown Occasionally, not all the CH2 groups (called methylenes) are shown. CH2 is shown once in parentheses, with a subscript indicating the number of methylene units strung together. CH3CH2CH2CH2CH2CH3 = CH3(CH2)4CH3 © 2017 Pearson Education, Inc.  12.4 Drawing Organic Structures A line structure (or line-angle structure) is a shorthand way of drawing structures in which carbon and hydrogen atoms are not shown. Guidelines for drawing a molecule as a line structure: 1. Each carbon–carbon bond is represented by a line. 2. Anywhere a line ends or begins, as well as any vertex where two lines meet, represents a carbon atom. 3. Any atom, other than another carbon or a hydrogen that is attached to a carbon must be shown. 4. Because a neutral carbon atom forms four bonds, all bonds not shown for any carbon are understood to be the number of carbon– hydrogen bonds needed to have the carbon form four bonds. © 2017 Pearson Education, Inc.  Worked Example 12.4 -Write condensed structures for all isomers that have the formula C6H14. or -The following structures all have the formula C7H16. Which of them represent the same molecule? a&b © 2017 Pearson Education, Inc.  Worked Example 12.5 - Convert the following condensed structures to line structures: OH Cl - Convert the following line structures to condensed structures: CH3 CH3CH2CCH2CH3 CH2CH3 © 2017 Pearson Education, Inc.  Hybridization Structure Bonding in Methane (CH4, an alkane)  Carbon is bonded to four hydrogen atoms  One s-orbital is combined with three p orbitals  The resultant sp3 hybridized carbon is tetrahedral  Each orbital is as far away as possible from the other orbitals H C H H H + sp3 s C H © 2017 Pearson Education, Inc. bond in methane  Bonding in Ethane  Contains two sp3 hybridized carbon atoms  Each carbon atom is tetrahedral H H H C C H H H sp3 - sp3 sigma bond + sp3 sp3 Sigma bond © 2017 Pearson Education, Inc.  12.6 Naming Alkanes There are four possible substitution patterns for carbons attached to four atoms. -primary (1°) carbon atom is a carbon attached to 1 other carbon. -secondary (2°) carbon atom is a carbon attached to 2 other carbons. -tertiary (3°) carbon atom is a carbon attached to 3 other carbons. -quaternary (4°) carbon atom is a carbon attached to 4 other carbons. Tertiary CH3 Primary H3C CH3 H3 C Secondary CH3 Quaternary © 2017 Pearson Education, Inc.  12.6 Naming Alkanes Learning Objective: Name an alkane given its structure and draw an alkane given its name. The system of naming (nomenclature) was devised by the International Union of Pure and Applied Chemistry (IUPAC). In the IUPAC system, a chemical name has three parts: – The prefix specifies the location of functional groups and other substituents. – The parent tells how many carbon atoms are present in the longest continuous chain. – The suffix identifies to which family the molecule belongs. © 2017 Pearson Education, Inc.  12.6 Naming Alkanes © 2017 Pearson Education, Inc.  12.6 Naming Alkanes -Substituent: An atom or group of atoms attached to a parent compound -Straight-chain alkanes are named by counting the number of carbon atoms and adding the family suffix -ane. -Straight-chain alkanes have no substituents, so prefixes are not needed. Substituents such as -CH3 and -CH2CH3, that branch off the main chain are called alkyl groups. An alkyl group is the part of the alkane that remains when a hydrogen atom is removed. Substituent Substituent © 2017 Pearson Education, Inc.  Alkyl group Alkane -H Alkyl group Examples: Methane -H Methyl -H CH3CH2 ethyl CH3CH3 Propyl © 2017 Pearson Education, Inc.  Commonly used alkyl groups H3C 1) Isopropyl C H3C H H3C 2) Isobutyl C-CH2 H3C H CH3 3) tert-butyl H3C C Each has four carbons CH3 from CH3CH2CH2CH3 4) sec-butyl H3C C CH2CH3 Butane H tert = tertiary, sec = secondary © 2017 Pearson Education, Inc.  12.6 Naming Alkanes The situation is more complex for larger alkanes. © 2017 Pearson Education, Inc.  12.6 Naming Alkanes © 2017 Pearson Education, Inc.  12.6 Naming Alkanes Branched-chain alkanes can be named by following four steps: 1. Name the main chain. Find the longest continuous chain of carbons, and name the chain according to the number of carbon atoms it contains. The longest chain may not be immediately obvious. 2. Number the carbon atoms in the main chain, beginning at the end nearer the first branch point. © 2017 Pearson Education, Inc. 3. Identify the branching substituents, and number each according to its point of attachment to the main chain. 2-methylpentane If there are two substituents on the same carbon, assign the same number to both. There must always be as many numbers in the name as there are substituents 3-ethyl-3-methylhexane 4. Write the name as a single word, using: hyphens (-) to separate the numbers from the different prefixes commas (,) to separate numbers if necessary. -If two or more different substituents are present, cite them in alphabetical order. -If two or more identical substituents are present, use one of the prefixes di-, tri-, tetra-, and so forth (but do not use them for alphabetizing purposes). © 2017 Pearson Education, Inc.  12.6 Naming Alkanes © 2017 Pearson Education, Inc.  Worked Example 12.9 What is the IUPAC name of the following alkanes? 2,5-dimethylheptane 4-isopropyl-3,4-dimethyloctane (d) (c) 2, 5-Dimethylheptane 4-Isopropyl-3, 4-Dimethyloctane © 2017 Pearson Education, Inc.  Worked Example 12.10 -Identify each carbon atom in the following molecule as primary, secondary, tertiary, or quaternary. (5) primary, (2) secondary, (1) tertiary, (1) quaternary. -Draw condensed and line structures for the following IUPAC names: CH3 a) 2,3-Dimethylpentane CH3CHCHCH2CH3 CH3 b) 3-Ethylheptane c) 4-tert-Butylheptane © 2017 Pearson Education, Inc.  12.7 Properties of Alkanes Alkanes contain only nonpolar C–C and C–H bonds. The only intermolecular forces influencing them are weak London dispersion forces. -The effect of London dispersion forces in alkanes is shown in the regularity with which the melting and boiling points of straight-chain alkanes increase with molecular size. The boiling points of straight chain alkanes rise as the chain length increases due to the larger surface area and the increased van der Waals attractions. © 2017 Pearson Education, Inc.  As branching increases, boiling point decreases Formula Boiling point (B.P) Despite having the same molecular IUPAC name --------------------------------------------------------------------------------------------------------------- weight, the boiling point decreases Pentane CH3(CH2)3CH3 36 as you go down from n-pentane to CH3 2-methylbutane H C CH2CH3 28 2,2-dimethylpropane. CH3 CH3 2,2-dimethylpropane H3C C CH3 10 CH3 -The first four alkanes, methane, ethane, propane, and butane, are gases at room temperature and pressure. -Alkanes with 5–15 carbon atoms are liquids. -Alkanes with 16 or more carbon atoms are generally low-melting, waxy solids. -Odorless or mild odor, colorless -Nonpolar, insoluble in water but soluble in nonpolar organic solvents, less dense than water -Flammable, otherwise not very reactive © 2017 Pearson Education, Inc.  12.8 Reactions of Alkanes Learning Objectives: -Determine the basic reactions of alkanes. -Draw the isomeric products formed during the halogenation of simple alkanes. Alkanes react with: oxygen (combustion) halogens (halogenation). Combustion The reaction of an alkane with oxygen is called combustion, an oxidation reaction that commonly takes place in a controlled manner in an engine or furnace. Carbon dioxide and water are the products of complete combustion of any hydrocarbon, and a large amount of heat is released. © 2017 Pearson Education, Inc.  12.8 Reactions of Alkanes Halogenation Halogenation is the replacement of an alkane hydrogen by a chlorine or bromine initiated by heat or light. The process is known as “free radical halogenation” and occurs in a stepwise manner. In a halogenation reaction, only one H at a time is replaced. If allowed to react for a long enough time, all Hs will be replaced with halogens. Complete chlorination of methane yields carbon tetrachloride: © 2017 Pearson Education, Inc.  12.8 Reactions of Alkanes Free Radical Halogenation: Termination Steps: © 2017 Pearson Education, Inc.  Worked Example 12.12 (a) Draw all singly chlorinated isomers obtained upon the reaction of pentane with chlorine. (b) Draw all singly brominated (monobrominated)isomers obtained upon the reaction of 2-methylbutane with bromine. © 2017 Pearson Education, Inc.  12.9 Cycloalkanes Learning Objective: Identify a cycloalkane from its structure. Cycloalkane: An alkane that contains a ring of carbon atoms To form a closed ring requires an additional C–C bond and the loss of 2 H atoms. The general formula for cycloalkanes is CnH2n. Compounds of ring sizes from 3 through 30 and beyond have been prepared in the laboratory. The C–C–C bond angles in cyclopropane are 60° and in cyclobutane are 90°(much less than the ideal 109.5° tetrahedral angle). They are less stable and more reactive than other cycloalkanes. © 2017 Pearson Education, Inc.  12.9 Cycloalkanes The C–C–C bond angles in cyclopentane and cyclohexane are near ideal. Both cyclopentane and cyclohexane rings are stable, and many naturally occurring and biochemically active molecules, such as steroids, contain such rings. © 2017 Pearson Education, Inc.  12.9 Cycloalkanes Properties of Cycloalkanes Cyclic and acyclic alkanes are similar in many of their properties. Cyclopropane and cyclobutane are gases at room temperature, whereas larger cycloalkanes are liquids or solids. Cycloalkans are nonpolar, insoluble in water, and flammable. Because of their cyclic structures, cycloalkane molecules are more rigid and less flexible than their open-chain counterparts. Rotation is not possible around the carbon–carbon bonds in cycloalkanes without breaking open the ring. This property is known as restricted rotation. © 2017 Pearson Education, Inc.  12.10 Drawing and Naming Cycloalkanes Learning Objective: Name a cycloalkane given its structure and draw a cycloalkane given its name. Line structures are used almost exclusively in drawing cycloalkanes. Polygons are used for the cyclic parts of the molecules. © 2017 Pearson Education, Inc.  12.10 Drawing and Naming Cycloalkanes Cycloalkanes are named by a straightforward extension of the rules for naming open-chain alkanes: STEP 1: Use the cycloalkane name as the parent. If there is only one substituent on the ring, it is not necessary to assign a number because all ring positions are identical. STEP 2: Identify and number the substituents. Start numbering at the group that has alphabetical priority, and proceed around the ring in the direction that gives the second substituent the lower possible number. © 2017 Pearson Education, Inc.  Worked Example 12.13 -What is the IUPAC name of the following cycloalkane? 1-isopropyl-3-methylcyclohexane -Draw a line structure for 1,3-dimethylcyclohexane. © 2017 Pearson Education, Inc.  Concept Map © 2017 Pearson Education, Inc.