Chapter 2 Alkanes: The Nature of Organic Compounds Suggested Problems: 29,31,34,36, 38,39,43,45-7,54,56, 68

2.1 Functional Groups Functional group - collection of atoms within a molecule that have a characteristic chemical behavior The group reacts in a typical way, generally independent of the rest of the molecule For example, the double bonds in simple and complex alkenes react with bromine in the same way

Functional Groups with Multiple Carbon–Carbon Bonds Alkenes have a C=C double bond Alkynes have a C≡C triple bond Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring

Functional Groups with Carbon Singly Bonded to an Electronegative Atom

Functional Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

Survey of Functional Groups

Survey of Functional Groups

Worked Example Identify the functional groups in the molecule of methionine Solution: Functional groups

2.2 Alkanes and Alkyl Groups: Isomers Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) Connecting carbons can lead to large or small molecules The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n Alkanes are saturated with hydrogen (no more H’s can be added) They are also called aliphatic compounds

Alkane Isomers CH4 = methane, C2H6 = ethane, C3H8= propane The molecular formula of an alkane with more than three carbons can give more than one structure based on different connections of the carbons C4 (butane) = butane and isobutane C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane Alkanes with each C connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C connected to 3 or 4 C’s are branched-chain alkanes Branched chain alkanes have the same formula as normal alkanes - CnH2n+2 – just not linear

Worked Example Draw structures of the five isomers of C6H14 Solution: The six-carbon straight-chain skeleton is drawn Draw a five-carbon chain, identifying different types of carbon atoms on the chain, and adding a –CH3 group to each of the different types of carbons, generating two skeletons, etc… Alkanes and alkane isomers

Constitutional Isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers They must have the same molecular formula to be isomers Compounds other than alkanes can be constitutional isomers of one another

Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of extended form A condensed structure does not show bonds but lists all atoms, such as CH3CH2CH2CH3 (butane) CH3(CH2)2CH3 (butane) Structural formulas

Naming Straight Chain Alkanes Note the names: Parent name followed by suffix (ane)

Alkyl Groups Alkyl group – remove one H from an alkane (constitutes a part of a structure) General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) Name: replace -ane ending of alkane with –yl ending -CH3 is “methyl” (from methane) -CH2CH3 is “ethyl” from ethane

Alkyl Groups (Continued) hexyl, heptyl, octyl, nonyl, decyl

Types of Alkyl Groups Classified by the connection site a carbon at the end of a chain (primary C) a carbon in the middle of a chain (secondary C) a carbon with three carbons attached to it (tertiary C)

Alkyl Groups Alkyl groups

2.3 Naming Branched-Chain Alkanes Compounds are given systematic names by a process that uses Follows specific rules 1) Find parent hydrocarbon chain

Naming Alkanes (Continued) 3,4,7 3,6,7 3,4 4,5 Begin numbering from side that affords smallest numbers

Naming Alkanes (Continued) Write compound name as single word

Naming Alkanes (Continued) Name a complex substituent as though it were a compound itself

Worked Example Give IUPAC names for the following compounds: a) b) Solution: a) 2,4-Dimethylpentane b) 2,2,5-Trimethylhexane Naming alkanes

Worked Example Give the IUPAC name for the following hydrocarbon and convert the drawing into a skeletal structure Naming alkanes

Worked Example Solution: Naming alkanes

2.4 Properties of Alkanes Called paraffins (low affinity compounds) because they do not react with most chemicals Used as oils, lubricants, waxes They will burn in a flame, producing carbon dioxide, water, and heat They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

Physical Properties of Alkanes Boiling points and melting points increase as size of alkane increases Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points Properties of alkanes

2.5 Conformations of Ethane Stereochemistry concerned with the 3-D aspects of molecules  bonds are cylindrically symmetrical Rotation is possible around C-C bonds in open-chain molecules H’s staggered H’s eclipsed

Conformers Conformation - Different arrangement of atoms resulting from bond rotation Conformations can be represented in 2 ways:

Conformers We do not observe perfectly free rotation There is a barrier to rotation, and some conformers are more stable than others Staggered- most stable: all 6 C-H bonds are as far away as possible Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other What is true for ethane is also true for higher alkanes, staggered arrangements are more favorable.

A Graph of Potential Energy Versus Bond Rotation in Ethane Conformations of ethane

Staggered vs Eclipsed Staggered Eclipsed Most stable Least stable

2.6 Drawing Chemical Structures Drawing every bond in an organic molecule can become tedious. Several shorthand methods have been developed to write structures. Condensed structures don’t have C-H or C-C single bonds shown. They are understood. e.g.

Drawing Structures (Continued) 3 General Rules for Line-Bond Structures: 1) Carbon atoms aren’t usually shown. Instead a carbon atom is assumed to be at each intersection of two lines (bonds) and at the end of each line. 2) Hydrogen atoms bonded to carbon aren’t shown. 3) Atoms other than carbon and hydrogen are shown.

Drawing Structures (Continued) Carvone has the following structure. Tell how many hydrogens are bonded to each carbon, and give the molecular formula for carvone. C10H14O 1 2 3

2.7 Cycloalkanes Many organic compounds contain rings of carbon atoms e.g. Prostaglandins Steroids

Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons Have the general formula (CnH2n) Each ring takes away two hydrogens from CnH2n+2

Naming Cycloalkanes 1) Find the parent. # of carbons in the ring. 2) Number the substituents 3) Use smallest numbers

2.8 Cis-Trans Isomerism in Cycloalkanes Cycloalkanes are less flexible than open-chain alkanes Much less conformational freedom in cycloalkanes

Cis-Trans Isomerism in Cycloalkanes (Continued) Because of their cyclic structure, cycloalkanes have 2 sides as viewed edge-on “top” side “bottom” side There are two different 1,2-dimethylcyclopropane isomers Cis – same side trans – opposite sides

Cis-Trans Isomerism in Cycloalkanes (Continued) Stereoisomerism Compounds which have their atoms connected in the same order but differ in 3-D orientation

Worked Example Draw the structures of the following molecules: a) trans-1-Bromo-3-methylcyclohexane b) cis-1,2-Dimethylcyclobutane Solution: Cis-trans isomerism in cycloalkanes

Worked Example b) cis-1,2-Dimethylcyclobutane Cis-trans isomerism in cycloalkanes

2.9 Conformations of Some Cycloalkanes Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze Angle strain – deviation of bond angles from 109.5o Torsional strain – strain from eclipsing interactions

2.9 Conformations of Some Cycloalkanes Angle Strain Torsional Strain

Conformations of Cyclohexane Substituted cyclohexanes occur widely in nature The cyclohexane ring is free of angle strain and torsional strain The conformation has alternating atoms in a common plane and tetrahedral angles between all carbons This is called a chair conformation

How to Draw Cyclohexane Step 1 Draw two parallel lines, slanted downward and slightly offset from each other. This means that four of the cyclohexane carbons lie in a plane. Step 2 Place the topmost carbon atom above and to the right of the plane of the other four, and connect bonds. Step 3 Place the bottom most carbon atom below and to the left of the plane of the middle four, and connect the bonds. Note that the bonds to the bottom most carbon atom are parallel to the bonds of the top most carbon.

2.10 Axial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring

Drawing the Axial and Equatorial Hydrogens

Worked Example Draw two different chair conformations of trans- 1,4-dimethylcyclohexane Label all positions as axial or equatorial Solution: Methyl substituents are either both axial or both equatorial trans-1,4-dimethylcyclohexane

Worked Example Draw two chair conformations each for cis- and trans-1-isopropyl-4-methylcyclohexane. Of these which is the most stable?

2.11 Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

Let’s Work a Problem Draw as many compounds as you can that are alcohols with the formula, C4H8O:

Answer The safest approach to answer this question would be to draw out all straight-chain isomers, then proceed next to the simplest branched structures and so on. 1-butanol 2-butanol isobutyl alcohol tert-butyl alcohol

Let’s Work a Problem Draw two constitutional isomers of cis-1,2-dibromocyclopentane?

Answer First, we need to understand what constitutional isomer means…the #’s of atoms, and types of atoms are the same, just the arrangement may be different. We have a 5 carbon cyclic alkane, so we can only have a case when we have a 1,2- or a 1,3- dibromo linkage, as these links will be symmetrical with respect to middle carbon.