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Chapter 3. An Introduction to Organic Compounds Learning objectives: 1. Name alkanes, cycloalkanes, and alkyl halides. 2. Write the structures of constitutional (or structural) isomers of alkanes. 3. Draw Newman projections of alkanes in staggered and eclipsed conformations, and arrange the order of stability for these conformations (conformational isomers). 4. Identify cis and trans relationship for the substituents on cycloalkanes. 5. Draw chair conformation of cyclohexane with unambiguous representation of axial and equatorial substituents. 6. Complete the equilibrium of two chair conformational isomers for a substituted cyclohexane, indicate the change for the relative positions of axial and equatorial substituents, and reason the stability between these two isomers. 7. Know the classification of alkyl halides, alcohols, and amines. Molecular model kit will be very helpful for learning the material in this chapter. 3.1 3.2 3.3 3.4 3.5 Nomenclature of Alkanes and Cycloalkanes Nomenclature and Classification of alkyl halides Alcohols, Amines and Ethers Structures and Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and Amines Conformation of Alkanes and Cycloalkanes 3.1 Nomenclature of Alkanes and Cycloalkanes A. Things to remember Know the names for alkanes from n=1 to 10. When viewed as substituents: alkanes become alkyl (R) groups. Know the names for alkyl groups from n=1 to 10. Name Formula Name of Substituent Structure of Substituent Methane CH4 Methyl CH3- Ethane CH3CH3 Ethyl CH3CH2- Propyl CH3CH2CH2-, npropyl (CH3)2CH-, isopropyl Propane CH3CH2CH3 1 Butane CH3(CH2)2CH3 Butyl CH3CH2CH2CH2-, nbutyl (CH3)2CHCH2-, isobutyl CH3CH2CH(CH3)-, sbutyl (CH3)3C-, t-butyl Pentane CH3(CH2)3CH3 Pentyl - Hexane CH3(CH2)4CH3 Hexyl - Heptane CH3(CH2)5CH3 Heptyl Octane CH3(CH2)6CH3 Octyl Nonane CH3(CH2)7CH3 Nonyl Decane CH3(CH2)8CH3 Decyl CH3(CH2)9CH3 - CH3(CH2)10CH3 - Undecane Dodecane B. Skeletal Structures Know the meaning of dashed and wedged bonds 2 propyl (n-propyl) isopropyl butyl (n-butyl) isobutyl sec-butyl ((s-butyl) tert-butyl (t-butyl) C. The IUPAC System (Systematic name) (i) Identify the longest carbon chain. (ii) Locate the substituent with the lowest number on the parent (main) chain. If equivalent positions are encountered, assign the substituent with higher alphabetical order a higher priority (lower in numbering). (iii) Place prefix of substituents according to their position on the main chain. Use di-, tri, tetra-, penta-, hexa- and so on for identical substituents. (iv) Arrange the substituents, excluding di-, tri-, tetra-, penta-, hexa- and so on, in alphabetical order. Examples: (i) 3 (ii) (iii) (iv) (v) (vi) (vii) (viii) 4 (ix) (x) (xi) D. Structural isomers Structural (constitutional) isomers: Compounds with the same molecular formula but with different order of attachment (connectivity) of their atoms. Examples: (i) Butane (C4H10) Normal butane (n-butane) CH3CH2CH2CH3 H H H H H C C C C H H H H 5 H 2-Methylpropane or isobutane (iso-butane or i-butane) H CH3CH(CH3)CH3 H H H C H C C C H H H H H (ii) Pentane (C5H12) CH3CH2CH2CH2CH3 CH3CH(CH3)CH2CH3 E. The Nomenclature of Cycloalkanes General formula: CnH2n (i) 6 CH3C(CH3)2CH3 (ii) (iii) (iv) (v) (vi) 7 F. Classification of Carbon and Hydrogen Atoms This classification cannot be used for alkenes (C=C) or alkynes (CΞC). G. Self-assessment Questions Can you provide the systematic names for alkanes and cycloalkanes? Can you identify primary, secondary, tertiary and quaternary carbons? 8 3.2 Nomenclature and Classification of Alkyl Halides A. Systematic name Use the name of alkanes and treat halides as substituents. B. Common Names Use the name of alkyl groups. Examples: 9 C. Self-assessment Questions Can you provide the systematic and common names for alkyl halides? Can you identify primary, secondary and tertiary alkyl halides? 3.3 Alcohols, Amines and Ethers A. Classification of Alcohols 10 B. Classification of Amines 11 C. Ethers D. Self-assessment Questions Can you identify primary, secondary and tertiary alkyl alcohols? Can you identify primary, secondary and tertiary amines, and quaternary ammonium salts? Can you recognize an ether? 12 3.4 Structures and Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and Amines A. Structures of Alkanes non-polar, induced dipole-induced dipole interaction (van der Waals force) B. Structures of Haloalkanes Dipole–dipole interaction 13 C. Structures of Ethers Dipole–dipole interaction D. Structures of Alcohols Hydrogen bonding 14 E. Structures of Amines Hydrogen bonding F. Intermolecular Interactions Know induced-dipole-induced-dipole interaction (van der Waals interaction) Know hydrogen bond 15 G. Comparison of Physical Properties (i) Boiling point Boiling points of alkanes increase with increasing molecule weight. (ii) Solubility in water The presence of hydrogen bond or polar groups H. Self-assessment Questions Can you recognize molecules that have interactions through induced-dipoleinduced-dipole (van der Waals), dipole-dipole and hydrogen bond? Can you compare the b.p. and solubility of molecules based on their structures? 16 3.5 Conformation of Alkanes and Cycloalkanes A. Constitutional Isomers Have Different Physical Properties B. Conformation: result of single bond rotation Conformational isomer: same atomic connectivity but different spatial arrangements of atoms. H H H H I I H H H H C II H C. Newman Projection (i) ethane H H HH H H H H H H H H II I Staggered conformation Eclipsed conformation 17 H C H II C H C (ii) butane CH3 H H H H H3C CH3 HCH3 H CH3 H H H H H H III II I CH3 H3CCH3 H H H H IV Steric strain (steric hindrance) If atoms within a molecule are brought too close together, there is an associated cost in energy due to overlapping electron clouds Anti-relationship 18 D. Conformers of Cycloalkanes (i) Ring strain and stability 19 (ii) Conformation of cyclohexane Know how to draw chair conformation of cyclohexane. Know how to show axial and equatorial positions of cyclohexane. twisted Chair H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H 20 Equatorial position Axial position E. Conformers of monosubstituted cyclohexanes H H H H C H H H H H C H H H H H H H 1,3-diaxial interaction (diaxial interaction, axial-axial interaction) 21 F. Cis and trans Isomers of Disubstituted Cycloalkanes Know cis - trans isomerism in cycloalkanes Cis and trans isomers are stereoisomers. Stereoisomers: same molecular formula and atom connectivity but different in the three-dimensional orientations of atoms in space (i) cyclopentane H H H H CH3 H H H H3C H H H H H CH3 H3C H H H H (ii) cyclohexane CH3 H H H3C H H3C H H H3C H H H ring flip ring flip 22 (iii) More examples: H3C CH3 Br Cl ring flip H3C ring flip CH3 C(CH3)3 Br ring flip ring flip 23 G. Fused Cyclohexane Rings trans-fused rings cis-fused rings H. Self-assessment Questions Can you explain why cyclopropane has strong ring strain? Can you draw Newman projections of alkanes in staggered and eclipsed conformations, and arrange the order of stability for these conformations (conformational isomers)? Can you identify cis and trans isomers for disubstituted cycloalkanes? Can you draw chair conformation of cyclohexane with unambiguous representation of axial and equatorial substituents? Can you complete the equilibrium of two chair conformational isomers for a substituted cyclohexane, indicate the change for the relative positions of axial and equatorial substituents, and reason the stability between these two isomers? 24 Homework for Chapter 3 (optional) Name: A number: 25