Essential Organic Chemistry Paula Yurkanis Bruice Chapter 3 An Introduction to Organic Compounds: Nomenclature, Physical Properties, and Representation of Structure

Alkanes Saturated hydrocarbons (Aliphatic) Hydrocarbons – Contain only C and H atoms. Saturated – Only single bonds. Aliphatic – “Fat” like. Straight-chain and branched-chain alkanes Can be acyclic (no rings) or cyclic (cycloalkanes).

Alkanes

Alkanes

Isomerism Consider C4H10 These structures are constitutional isomers

Isomerism Consider C5H12 These structures are constitutional isomers

Isomerism Consider C6H14 These structures are constitutional isomers

Isomerism Isomerism – The phenomenon whereby certain chemical compounds have structures that are different although the compounds possess the same elemental composition. Isomers – Two or more chemical substances having the same elementary composition and molecular weight but differing in structure.

3.1 Nomenclature of Alkyl Substituents Names and Formulas of Alkyl Groups: Formula Name CH3- methyl CH3CH2CH2CH2- butyl CH3CH2- ethyl (CH3)2CHCH2- isobutyl CH3CH2CH2- propyl CH3CH2CH(CH3)- sec-butyl (CH3)2CH- isopropyl (CH3)3C- tert-butyl

Nonsystematic names; Common names

Primary (1o) carbon – a carbon that is bonded to only one other carbon. Secondary (2o) carbon– a carbon that is bonded to two other carbons. Tertiary (3o) carbon– a carbon that is bonded to three other carbons.

Nonsystematic names; Common names Figure: 03-00-15UN Title: Iso-alkyl groups Caption: Whenever the iso- prefix is used, the iso- structural unit will be at one end of the structure and the group replacing the hydrogen is at the other end. Notes:

Names of Some Alkyl Groups Figure: 03-00-16UN.T2 Title: Table 3.2 Names of some alkyl groups Caption: The condensed structures of various alkyl groups for the first 6 alkanes. Notes: The condensed structures of various alkyl groups for methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, hexyl, isohexyl.

3.2 IUPAC Nomenclature of Alkanes 1. Determine the number of carbons in the longest continuous carbon chain as the parent hydrocarbon. 2. Number the chain so that the substituent gets the lowest possible number. Numbers are used only for systematic names, never for common names. A number and a word are separated by a hyphen

3.2 IUPAC Nomenclature of Alkanes 3. Substituent are listed in alphabetical order. Numbers are separated by a comma. When the same alkyl group branch chain occurs more than once, indicate this repetition by a prefix (di, tri, tetra, and so forth). di, tri, tetra, sec, and so on are ignored in alphabetizing. iso and cyclo are not ignored in alphabetizing. 4. When both directions lead to the same lowest number for one of the substituents, the direction that gives the lowest possible number to one of the remaining substituents is chosen.

3.2 IUPAC Nomenclature of Alkanes 5. If the same substituent numbers are obtained in both directions, the first group cited receives the lower number. 6. If a compound has two or more chains of the same length, the parent hydrocarbon is the chain with the greatest number of substituents.

Examples

Examples

Examples Note: Number the chain so that the substituents get the lowest possible numbers.

Examples Caution: Be careful to choose the longest chain as the parent alkane.

Examples Note: Substituents are listed in alphabetical order.

Draw the Compounds 3-ethylpentane 2,2,4-trimethylpentane

3.3 IUPAC Nomenclature of Cycloalkanes Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

IUPAC Nomenclature of Cycloalkanes In the case of a cycloalkane with an attached alkyl substituent, the ring is the parent hydrocarbon. There is no need to number the position of a single substituent on a ring. If the ring has two different substituents, they are cited in alphabetical order and the number 1 position is given to the substituent cited first. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Cycloalkanes Cyclopropane Methylcyclohexane

Cycloalkanes 1-Cyclobutylhexane or Hexylcyclobutane

Cycloalkanes 1-Ethyl-2-methylcyclohexane

Name the Following Compounds Methylcyclopropane 1,1-Dimethylcyclohexane 1,2-Dimethylcyclopentane 3-Cyclopropylpentane

Figure: 03-00-32UN Title: Skeletal structures of alkanes Caption: Skeletal structures of noncyclic alkanes. Notes: These are line drawings that are generally used when representing alkanes. It is understood that each vertex is a carbon atom and that there are the appropriate number of hydrogens attached to each.

3.4 IUPAC Nomenclature of Alkyl Halides Common name: alkyl group + halogen, with the “ine” ending replaced by “ide” (fluoride, chloride, bromide, iodide) IUPAC name: substituted alkanes, with the substituent prefix for the halogens end with “O” (fluoro, chloro, bromo, iodo) Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

IUPAC Nomenclature of Alkyl Halides Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

3.5 Classification of Alkyl Halides, Alcohols, and Amines The number of alkyl groups attached to the carbon to which the halogen is bonded determines whether an alkyl halide is primary, secondary, or tertiary. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Classification of Alkyl Halides, Alcohols, and Amines The number of alkyl groups attached to the carbon to which the OH group is attached determines whether an alcohol is primary, secondary, or tertiary. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Classification of Alkyl Halides, Alcohols, and Amines The number of alkyl groups attached to the nitrogen determines whether an amine is primary, secondary, or tertiary. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

3.6 Structures of Alkyl Halides, Alcohols, Ethers, and Amines Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Structures of Alkyl Halides, Alcohols, Ethers, and Amines Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Structures of Alkyl Halides, Alcohols, Ethers, and Amines Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Structures of Alkyl Halides, Alcohols, Ethers, and Amines Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

3.7 Physical Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and Amines Boiling Points (bp) – the temperature at which the liquid form of the compound become a gas (vaporizes). Induced-dipole-induced-dipole interactions van der Waals forces – in order for an alkane to boil, the van der Waals forces must be overcome. The van der Waals forces that hold alkane molecules together depends on the area of contact between the molecules. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Boiling Points of Alkanes Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n. Boiling points increase with increasing molecular weight within a homologous series of alkanes.

Boiling Points of Alkanes Branching decreases the area of contact between molecules. If two alkanes have the same molecular weight, the more highly branched alkane will have a lower boiling point. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Boiling Points of Ethers, Alcohols, and Amines Boiling points increase with increasing molecular weight within a homologous series. Ethers generally have higher boiling points than alkanes of comparable molecular weight because both van der Waals forces and dipole-dipole interactions must be overcome. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Boiling Points of Ethers, Alcohols, and Amines 3. Alcohols have much higher boiling points than alkanes or ethers of comparable molecular weight because, in addition to van der Waals forces and dipole-dipole interactions, hydrogen bonds have to be broken as well. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Boiling Points of Ethers, Alcohols, and Amines 4. Primary and secondary amines also form hydrogen bonds, so these amines have higher boiling points than alkanes with similar molecular weights. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Boiling Points of Alkyl Halides 5. Both van der Waals forces and dipole-dipole-interactions must be overcome for alkyl halides to boil. 6. As the halogen atom increases in size, the size of its electron cloud increases, and the larger the electron cloud, the stronger are the van der Waals interactions. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Melting Points of Alkanes Melting Points (mp) – the temperature at which a solid is converted into a liquid. The increase in mp is less regular than that in bp because packing influences the mp of a compound. Packing – a property that determines how well the individual molecules in a solid fit together in a crystal lattice. Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Melting Points of Alkanes Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n. Melting points increase with increasing molecular weight within a homologous series of alkanes.

Melting Points of Alkanes Figure: 03-00-30UN Title: Structures of cycloalkanes Caption: Structures of the cycloalkanes cyclopropane, cyclobutane, cyclopentane, cyclohexane. Notes: Note the molecular formula for a cycloalkane is CnH2n.

Solubility of Alkanes Solubility – “Like dissolves like” Alkanes are nonpolar, hydrophobic They are soluble in nonpolar solvents and insoluble in water.

3.8 Conformations of Alkanes: Rotation About Carbon-Carbon Bonds The different spatial arrangements of the atoms that result from rotation about a single bond are called “conformations”. A specific conformation is called a conformer.

Conformations of Alkanes When rotation occurs about the carbon-carbon bond of ethane, two extreme conformations can result- a staggered conformations and an eclipsed conformation. In a Newman projection, you are looking down the length of a particular C-C bond.

Conformations of Alkanes Ethane Staggered conformation Newman projection: =

Conformations of Alkanes Ethane Eclipsed conformation Newman projection: = ~3 kcal/mole higher in energy than staggered conformation

Conformations of Alkanes The electrons in a C-H bond will repel the electrons in another C-H bond if the bonds get too close to each other. This is called “torsional strain”. Staggered conformer: the most stable; at energy minima Eclipsed conformer: the least stable ; at energy maxima

Conformations of Alkanes Steric strain is the strain (extra energy) put on a molecule when atoms or groups are too close to one another.

3.9 Cycloalkanes: Ring Strain The deviation of the bond angle from the ideal bond angle causes strain called angle strain. Cyclopropane: 60o ; Cyclobutane: 90o. Cyclobutane: a bent molecule, not planar- to reduce torsional strain!

3.10 Conformations of Cyclohexane “Chair” conformation

Conformations of Cyclohexane “Chair” conformation Equatorial hydrogens

Figure: 03-05-05UN Title: Conformation of Cyclohexane Caption: The chair conformation of cyclohexane showing the positions of the equatorial bonds. Notes: The axial bonds are the other bonds shown but not highlighted.

Conformations of Cyclohexane “Chair” conformation Axial hydrogens

Figure: 03-05-04UN Title: Conformation of Cyclohexane Caption: The chair conformation of cyclohexane showing the positions of the axial bonds. Notes:

Figure: 03-06 Title: Chair Conformation of Cyclohexane Caption: The skeletal structure, Newman projection, and ball-and-stick models of the chair conformation of cyclohexane. Notes: All the bonds are staggered in these models.

Figure: 03-06-02UN Title: Chair Conformation of Cyclohexane Caption: The chair conformation of cyclohexane showing the positions of all the axial and equatorial bonds. Notes: Remember that the lower bonds are in front and the upper bonds are in back since this molecule is shown on edge.

Figure: 03-07 Title: Ring Flip of Cyclohexane Caption: The bonds that are axial in one chair conformer are equatorial in the other chair conformer. The bonds that are equatorial in one chair conformer are axial in the other chair conformer. Notes: This ring flip occurs very rapidly at room temperature.

Conformations of Cyclohexane “Boat” conformation

Conformations of Cyclohexane “Boat” conformation Steric interactions

Conformations of Cyclohexane “Boat” conformation Eclipsing

Conformations of Cyclohexane “Twist-boat” conformation “Half-chair” conformation

Conformations of Cyclohexane Figure: 03-08 Title: Conformations of Cyclohexane Caption: Conformations of cyclohexane (and their relative energies) during a ring flip. Notes: The chair conformations are the most stable. The twist-boat is the next most stable conformation.

3.11 Conformations of Monosubstituted Cyclohexanes Methylcyclohexane

Conformations of Monosubstituted Cyclohexanes Methylcyclohexane C H 3 steric interactions more stable

Conformations of Monosubstituted Cyclohexanes tert-Butylcyclohexane severe steric interactions much more stable

3.12 Conformations of Disubstituted Cyclohexanes cis-1,3-Dimethylcyclohexane severe steric interactions much more stable

Figure: 03-09-02UN Title: Geometric Isomers of 1,4-dimethylcyclohexane Caption: These are the cis and trans isomers of 1,4-dimethylcyclohexane. When the methyl groups are cis, one is in the axial position and the other is in the equatorial position. When they are trans, both are in the equatorial position. Notes: When the methyl groups are cis, one is in the axial position and the other is in the equatorial position. When they are trans, both are in the equatorial position.

Figure: 03-09-11UN Title: Ring Flip of cis-1,4-Dimethylcyclohexane Caption: When the ring flips between the two chair conformations the methyl group that was in the equatorial position becomes axial and the axial becomes equatorial. Notes: Both conformers are equally stable since at least one of the substituents is equatorial.

Figure: 03-09-12UN Title: Ring Flip of trans-1,4-Dimethylcyclohexane Caption: When the ring flips between the two chair conformations the methyl groups that were in the equatorial positions become axial. Notes: In this case the first conformation is the more stable conformation since both methyl groups are in the equatorial position.

3.13 Conformations of Fused Rings Figure: 03-09-15UN Title: Fused Rings Caption: Trans-fused rings are more stable than are cis-fused rings since all substituents are in the equatorial position. Notes: In the trans-fused ring both substituents are in the equatorial position, which is more stable.

Figure: 03-09-19UN Title: Structure of Cholesterol Caption: Structure of cholesterol. Notes: Cholesterol is the important component of cell membranes.