3. Organic Compounds: Alkanes and Their Stereochemistry Why this Chapter Alkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds Alkanes will be used to discuss basic approaches to naming organic compounds We will take an initial look at 3-D aspects of molecules

3.1 Functional Groups Functional group - collection of atoms at a site that have a characteristic behavior in all molecules where it occurs 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

Alkenes and Alkynes Alkenes contain a double bond between adjacent carbon atoms. Alkynes contain a triple bond. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Functional Groups with: Carbon Singly Bonded to an Electronegative Atom

Alcohols and Ethers An alcohol contains the hydroxyl (-OH) functional group. In an ether, an oxygen atom is bonded to two carbon atoms –C–O–C– . Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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

Aldehydes and Ketones An aldehyde contains a carbonyl group (C=O), which is a carbon atom with a double bond to an oxygen atom, attached to at least one hydrogen. In a ketone, the carbon of the carbonyl group is attached to two other carbon atoms. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Carboxylic Acids and Esters Carboxylic acids contain the carboxyl group, which is a carbonyl group attached to a hydroxyl group. O  — C—OH An ester contains the carboxyl group between carbon atoms. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Amines and Amides In amines, the functional group is a nitrogen atom. | —N — In amides, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Summary of Functional Groups

Survey of Functional Groups

Survey of Functional Groups

Functional Groups in Everyday Items Methyl amine (fish) Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check 1) CH3–CH2–CH2–OH 2) CH3–O–CH2–CH3 3) CH3–CH2–NH2 O O Classify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine. 1) CH3–CH2–CH2–OH 2) CH3–O–CH2–CH3 3) CH3–CH2–NH2 O O || || 4) CH3–C–OH 5) CH3–C–O–CH3

Solution 1) CH3─CH2─CH2─OH 2) CH3─O─CH2─CH3 alcohol ether Classify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine. 1) CH3─CH2─CH2─OH 2) CH3─O─CH2─CH3 alcohol ether 3) CH3─CH2─NH2 amine O O || || 4) CH3─C─OH 5) CH3─C─O─CH3 carboxylic acid ester

Learning Check What functional group is not present in the naturally occurring compound shown below? carboxylic acid amine primary alcohol secondary alcohol methyl group

Solution What functional group is not present in the naturally occurring compound shown below? carboxylic acid amine primary alcohol secondary alcohol methyl group

3.2 Alkanes and Alkane 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 can be added They are also called aliphatic compounds

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 atoms, such as CH3CH2CH2CH3 (butane) CH3(CH2)2CH3 (butane)

Expanded and Condensed Structures

Some Structures for Butane Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Line-Bond Formulas Because each C atom has a tetrahedral arrangement, the order of atoms is not a straight line, but a zigzag pattern. A line-bond formula abbreviates the carbon atoms and shows only the zigzag pattern of bonds from carbon atom to carbon atom.

Hexane has Six Carbon Atoms Is an alkane with 6 carbon atoms in a continuous chain. Has a “zig-zag” look because each carbon atom is at the center of a tetrahedron. Is represented by a ball-and-stick model as

Learning Check A. Write the condensed formula for: H H H H H H C C C C C H H H H H H B. What is its molecular formula? C. What is its name?

Solution CH3CH2CH2CH2CH3 C5H12 pentane A. Write the condensed formula for: H H H H H H C C C C C H H H H H H B. What is its molecular formula? C. What is its name? CH3CH2CH2CH2CH3 C5H12 pentane

Learning Check Write the condensed structural formula for A. ethane B. heptane

Solution Write the condensed structural formula for A. ethane CH3─CH3 B. heptane CH3─CH2─CH2─CH2─CH2─CH2─CH3

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 C4 (butane) = butane and isobutane C5 (pentane) = pentane, 2-methylbutane, and 2,2- dimethylpropane Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

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

Isomers of Butane Isomers Have the same molecular formula. Have different atom arrangements. Of butane (C4H10) consist of a straight chain and a branched chain each with 4 carbon atoms. branch

What kind of carbon is not present in the molecule below? Learning Check What kind of carbon is not present in the molecule below? primary secondary tertiary quaternary All 4 kinds of carbon are present

What kind of carbon is not present in the molecule below? Solution What kind of carbon is not present in the molecule below? primary secondary tertiary quaternary All 4 kinds of carbon are present

How many secondary hydrogens are there in the following molecule? Learning Check How many secondary hydrogens are there in the following molecule? 2 3 4 5 6

How many secondary hydrogens are there in the following molecule? Solution How many secondary hydrogens are there in the following molecule? 2 3 4 5 6

Which of the following choices is not an alkane? Learning Check Which of the following choices is not an alkane? C12H26 C10H20 C11H24 C16H34 C20H42

Which of the following choices is not an alkane? Solution Which of the following choices is not an alkane? C12H26 C10H20 C11H24 C16H34 C20H42

Learning Check A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for an alkane? 2° hydrogens 2° carbons 6 2 4 4 3 5 3 6 8 4

Solution A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for an alkane? 2° hydrogens 2° carbons 6 2 4 4 3 5 3 6 8 4

3.4 Naming Alkanes Compounds are given systematic names by a process that uses Follows specific rules Find parent hydrocarbon chain Carbons in that main chain are numbered in sequence Substituents are identified numbered Write compound name is single word Name a complex substituents as though it were a compound itself See specific examples in text

IUPAC Names for Simple Alkanes Use a prefix to indicate the number of carbons in a chain. End in –ane. The names of organic compounds are determined by the IUPAC rules (International Union of Pure and Applied Chemistry). The stem of the name states the number of carbon atoms in the carbon chain of the compounds. The suffix, in this case –ane, indicates the alkane family.

Learning Check A. Give the name of each compound: 1) CH3—CH3 2) CH3—CH2—CH3 3) CH3—CH2—CH2—CH2—CH2—CH3 B. Write the condensed structural formula of pentane.

Solution A. Give the name of each compound: 1) CH3—CH3 ethane 2) CH3—CH2—CH3 propane 3) CH3—CH2—CH2—CH2—CH2—CH3 hexane B. Write the condensed structural formula of pentane. CH3—CH2—CH2—CH2—CH3

3.3 Alkyl Groups Alkyl group – remove one H from an alkane (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

Types of Alkyl groups Classified by the connection site (See Figure 3.3) a carbon at the end of a chain (primary alkyl group) a carbon in the middle of a chain (secondary alkyl group) a carbon with three carbons attached to it (tertiary alkyl group)

Naming Substituents IUPAC Common methyl ethyl propyl n-propyl Isopropyl butyl n-butyl 1-methylpropyl sec-butyl 2-methylpropyl Isobutyl 1,1-dimethylethyl tert-butyl or t-butyl

Naming Substituents IUPAC Common pentyl n-pentyl or amyl 1-methylbutyl sec-pentyl 3-methylbutyl isopentyl or isoamyl or i-amyl 1,1-dimethylpropyl tert-pentyl or tert-amyl or t-amyl 2,2-dimethylpropyl neopentyl

Naming Substituents IUPAC Fluoro Chloro Bromo Iodo Hydroxy

Alkanes with Alkyl Groups methylpropane methyl groups 2,4-dimethylpentane

Learning Check Write three isomers of C5H12 and name each.

Solution Write three isomers of C5H12 and name each. A chain of 5 carbon atoms CH3─CH2─CH2─CH2─CH3 pentane A chain of 4 carbon atoms and 1 carbon branch CH3 │ CH3─CH─CH2─CH3 2-methylbutane A chain of 3 carbon atoms and 2 carbon branches CH3─C─CH3 2,2-dimethylpropane

What is the best IUPAC name for the following compound? Learning Check What is the best IUPAC name for the following compound? 2-ethyl-3-isopropylhexane 1-isopropyl-3-ethylhexane 4-isopropyl-5-ethylhexane 3-methyl-4-isopropylheptane 4-isopropyl-3-methylheptane

What is the best IUPAC name for the following compound? Solution What is the best IUPAC name for the following compound? 2-ethyl-3-isopropylhexane 1-isopropyl-3-ethylhexane 4-isopropyl-5-ethylhexane 3-methyl-4-isopropylheptane 4-isopropyl-3-methylheptane

What is the systematic name of the alkane shown below? Learning Check What is the systematic name of the alkane shown below? 3,7-dimethyl-4-propyloctane 2-methyl-5-(1-methylpropyl)octane 2-ethyl-6-methyl-3-propylheptane 3-methyl-4-(3-methylbutyl)heptane 2,6-dimethyl-5-propyloctane

What is the systematic name of the alkane shown below? Solution What is the systematic name of the alkane shown below? 3,7-dimethyl-4-propyloctane 2-methyl-5-(1-methylpropyl)octane 2-ethyl-6-methyl-3-propylheptane 3-methyl-4-(3-methylbutyl)heptane 2,6-dimethyl-5-propyloctane

Which of the following is not an isomer of hexane? Learning Check Which of the following is not an isomer of hexane? 2,2-dimethylbutane 2,2-dimethylpentane 3-methylpentane 2-methylpentane 2,3-dimethylbutane

Which of the following is not an isomer of hexane? Solution Which of the following is not an isomer of hexane? 2,2-dimethylbutane 2,2-dimethylpentane 3-methylpentane 2-methylpentane 2,3-dimethylbutane

How many constitutional isomers of C4H10O are alcohols? Learning Check How many constitutional isomers of C4H10O are alcohols? 1 2 3 4 5

How many constitutional isomers of C4H10O are alcohols? Solution How many constitutional isomers of C4H10O are alcohols? 1 2 3 4 5

3.5 Properties of Alkanes Called paraffins (low affinity compounds) because they do not react as most chemicals They will burn in a flame, producing carbon dioxide, water, and heat

Crude Oil The hydrocarbons in crude oil are Separated by boiling points. Heated to higher temperatures to produce gases that can be removed and cooled. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Combustion of Alkanes Alkanes Undergo combustion by reacting with oxygen to produce carbon dioxide, water, and energy. Are typically not very reactive due to strong C-C single bonds. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings alkane + O2 CO2 + H2O + energy

Learning Check Propane is used to provide heat for cooking or warming a room. Write a balanced equation for the complete combustion of propane. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Solution Propane is used to provide heat for cooking or warming a room. Write a balanced equation for the complete combustion of propane. Solution C3H8 + 5O2 3CO2 + 4H2O Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Write a balanced equation for the reaction for the complete combustion of: heptane ethane

Solution Write a balanced equation for the reaction for the complete combustion of: heptane C7H16. ethane C7H16 + 11O2  7CO2 + 8H2O 2C2H6 + 7O2  4CO2 + 6H2O

Properties of Alkanes They react with Halogens like Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

Learning Check Give the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light.

Solution CH3—CH2—CH2— Br 1-bromopropane; propyl bromide Br | Give the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light. CH3—CH2—CH2— Br 1-bromopropane; propyl bromide Br | CH3—CH—CH3 2-bromopropane; isopropyl bromide

Physical Properties 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

3.6 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

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

Torsional Strain 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

3.7 Conformations of Other Alkanes The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH3 interaction

Conformations of Other Alkanes • Conformational situation is more complex for larger alkanes • Not all staggered conformations has same energy, and not all eclipsed conformations have same energy

Conformations of Butane Anti conformation- methyl groups are 180˚ apart Gauche conformation- methyl groups are 60˚ apart Which is the most energetically stable?

Steric Strain Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow 11 6 3.8 6 4 4 4

Learning Check What is the correct order of stability, starting with the most strained, for the conformations shown below? A B, C A, C, B B, A, C C, A, B C, B, A

Solution What is the correct order of stability, starting with the most strained, for the conformations shown below? A B, C A, C, B B, A, C C, A, B C, B, A

Learning Check C6H14 has 5 isomers. How many of these isomers have at least one tertiary carbon? 1 2 3 4

Solution C6H14 has 5 isomers. How many of these isomers have at least one tertiary carbon? 1 2 3 4

Learning Check Which statement about bond rotations around C-C bonds in ethane and ethylene is correct? Rotation around σ bond is easy because the orbital overlap does not change with rotation. Rotation around π bond is easy because there are p orbitals available in all directions of space. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other. Rotation around π bond is hard because the electrons are delocalized.

Solution Which statement about bond rotations around C-C bonds in ethane and ethylene is correct? Rotation around σ bond is easy because the orbital overlap does not change with rotation. Rotation around π bond is easy because there are p orbitals available in all directions of space. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other. Rotation around π bond is hard because the electrons are delocalized.

Learning Check Which of the above represents the Newman projection of 2-bromo-3-chlorobutane? a b c d e

Solution Which of the above represents the Newman projection of 2-bromo-3-chlorobutane? a b c d e

Learning Check 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 0.0 kJ/mol 11.0 kJ/mol The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation (bent rod) must have? 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 0.0 kJ/mol 11.0 kJ/mol

Solution 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 0.0 kJ/mol 11.0 kJ/mol The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation (bent rod) must have? 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 0.0 kJ/mol 11.0 kJ/mol

Learning Check the energy of one gauche interaction What is the energy difference between the two lowest-energy conformations of 2,3-dimethylbutane? the energy of one gauche interaction the energy of two gauche interactions the energy of two H ↔ CH3 eclipsing interactions the energy of three H ↔ CH3 eclipsing interactions the energy of two CH3 ↔ CH3 eclipsing interactions

Solution the energy of one gauche interaction What is the energy difference between the two lowest-energy conformations of 2,3-dimethylbutane? the energy of one gauche interaction the energy of two gauche interactions the energy of two H ↔ CH3 eclipsing interactions the energy of three H ↔ CH3 eclipsing interactions the energy of two CH3 ↔ CH3 eclipsing interactions

Learning Check 16 kJ/mol 20 kJ/mol 26 kJ/mol 31 kJ/mol 37 kJ/mol The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have? 16 kJ/mol 20 kJ/mol 26 kJ/mol 31 kJ/mol 37 kJ/mol

Solution 16 kJ/mol 20 kJ/mol 26 kJ/mol 31 kJ/mol 37 kJ/mol The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have? 16 kJ/mol 20 kJ/mol 26 kJ/mol 31 kJ/mol 37 kJ/mol