Che 163 Introductory Organic Chemistry ALKANES Summer Quarter 2010

What is Organic chemistry?

What is Organic chemistry? The study of carbon and its compounds.

What is Organic chemistry? The study of carbon and its compounds. First we will concentrate on compounds just containing carbon and hydrogen, these compounds are called hydrocarbons.

Hydrocarbon Classification What is Organic chemistry? The study of carbon and its compounds. First we will concentrate on compounds just containing carbon and hydrogen, these compounds are called hydrocarbons. Hydrocarbon Classification Hydrocarbons Alkanes Cycloalkanes Alkenes Cycloalkenes Alkynes

Alkanes (saturated) hydrocarbons, or aliphatic hydrocarbons) General formula of CnH2n+2 Examples a. CH4 b. C2H6 c. C3H?

Alkanes General formula of CnH2n+2 Examples a. CH4 b. C2H6 c. C3H8 d. C4H?

General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Alkanes General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Draw Lewis Structures CH4 C2H6 C3H8

General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Alkanes General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Draw Lewis Structures CH4 C2H6 C3H8 D. Polarity? Polar or nonpolar?

General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Alkanes General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10 Draw Lewis Structures CH4 C2H6 C3H8 D. Polarity? Polar or nonpolar? Nonpolar

E. Draw three dimensional structures, bond angles and hybridization. Alkanes (Continued) E. Draw three dimensional structures, bond angles and hybridization. CH4 C2H6 C3H8 F. There are two different structures for C4H 10 Structure 1 Structure 2

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 => Primary = ? Secondary = Tertiary = Primary = Secondary = Tertiary =

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 => Primary = 2 Secondary = ? Tertiary = Primary = Secondary = Tertiary =

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertirary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 => Primary = 2 Secondary = 2 Tertiary = ? Primary = Secondary = Tertiary =

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 => Primary = 2 Secondary = 2 Tertiary = 3 Primary = ? Secondary = Tertiary =

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 Primary = 2 Secondary = 2 Tertiary = 3 Primary = 3 Secondary = ? Tertiary =

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 Primary = 2 Secondary = 2 Tertiary = 3 Primary = 3 Secondary = 0 Tertiary = ?

1. Primary (1◦) Carbon connected to one carbon atoms. Types of carbon 1. Primary (1◦) Carbon connected to one carbon atoms. 2. Secondary (2◦) Carbon connected to two carbon atoms. 3. Tertiary (3◦) Carbon connected to three carbon atoms. 4. How many primary, secondary, and tertiary carbons in the two different structures of C4H10 Primary = 3 Secondary = 0 Tertiary = 1 Primary = 2 Secondary = 2 Tertiary = 3

Constitutional Isomers (Structural Isomers) are different compounds of the same formula. The different structures from the previous slide for the formula C4H10 is an example of Constitutional isomers. How many isomers are there of an alkane containing five carbons (C5H10)?

NOMENCLATURE Common system Works best for low molecular weight hydrocarbons Steps to give a hydrocarbon a common name: Count the total number of carbon atoms in the molecule. Use the Latin root from the following slide that corresponds to the number of carbon atoms followed by the suffix “ane”. Unbranced hydrocarbons use the prefix normal, or n-, Branched hydrocarbons use specific prefixes, as shown on a subsequent slide

Latin Hydrocarbon Roots Latin Hydrocarbon Roots Examples Latin Hydrocarbon Roots Number of Carbons Latin Root 1 meth 2 eth 3 prop 4 but 5 pent 6 hex 7 hept 8 oct 9 non 10 dec 11 undec Latin Hydrocarbon Roots Number of Carbons Latin Root 12 dodec 13 tridec 14 tetradec 15 pentadec 16 hexadec 17 heptadec 18 octadec 19 nonadec 20 eicos 21 unicos 22 doicos H n-butane H isobutane H H H C H H H C C C H H H C H H H neopentane

2. Systematic System of Nomenclature (IUPAC) Find the longest continuous chain of carbon atoms. Use a Latin root corresponding to the number of carbons in the longest chain of carbons. Follow the root with the suffix of “ane” for alkanes Carbon atoms not included in the chain are named as substituents preceding the root name with Latin root followed by “yl” suffix. Number the carbons, starting closest to the first branch. Name the substituents attached to the chain, using the carbon number as the locator in alphabetical order. Use di-, tri-, etc., for multiples of same substituent. If there are two possible chains with the same number of carbons, use the chain with the most substituents.

Substituent Names (Alkyl groups)

Systematic Nomenclature continued. Which one?

Systematic Nomenclature continued. Which one? The one with the most number of substituents

Systematic Nomenclature continued. Which one? The one with the least number of substituents The top structure has four substituents and the bottom has three Substituents.

Systematic Nomenclature continued. Which one? The one with the least number of substituents The top structure has four substituents and the bottom has three Substituents. Name = ?

Systematic Nomenclature continued. Which one? The one with the least number of substituents The top structure has four substituents and the bottom has three Substituents. Name = ? heptane

Systematic Nomenclature continued. Which one? The one with the least number of substituents The top structure has four substituents and the bottom has three Substituents. Name = 3,3,5-trimethyl-4-propylheptane

Another Example: Name = 3-ethyl-2,6-dimethylheptane

Another Example: Name = 2,6-dimethyl-3-ethylheptane Notice substituents are in alphabetical order; di, tri, etc. do not participate in the alphabetical order

Line Structures A quicker way to write sturctures (Condensed Structure) methyl ethyl (A line structure of the above condensed structure) methyl

Complex Substituents If the branch has a branch, number the carbons from the point of attachment. Name the branch off the branch using a locator number. Parentheses are used around the complex branch name. 3 1 1 2 1-methyl-3-(1,2-dimethylpropyl)cyclohexane

Alkane Physical Properties Solubility: hydrophobic (not water soluble) Density: less than 1 g/mL (floats on water) Boiling points increase with increasing carbons (little less for branched chains) due to dispersion forces being larger. Melting points increase with increasing carbons (less for odd-number of carbons).

Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces.

Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

Reactions of Alkanes II. Cracking reaction I. Combustion reaction II. Cracking reaction III. Halogenation reaction (substitution reaction)

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H12 Step 2 react each isomer with chlorine Step 3 count the products

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H10 Step 2 react each isomer with chlorine Step 3 count the products

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H10 Step 2 react each isomer with chlorine Step 3 count the products

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H10 Step 2 react each isomer with chlorine Step 3 count the products

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H10 Step 2 react each isomer with chlorine Step 3 count the products

Which isomer of C5H12 has the most monochloro isomers? Sample problem: Which isomer of C5H12 has the most monochloro isomers? Problem solving process: Step 1 draw the isomers of C5H10 Step 2 react each isomer with chlorine Step 3 count the products Winner!

Conformers of Alkanes Structures resulting from the free rotation of a C-C single bond May differ in energy. The lowest-energy conformer is most prevalent. Molecules constantly rotate through all the possible conformations.

Ethane Conformers Staggered conformer has lowest energy. Dihedral angle = 60 degrees model H Newman projection Dihedral angle sawhorse

Ethane Conformers (2) Eclipsed conformer has highest energy Dihedral angle = 0 degrees =>

Conformational Analysis Torsional strain: resistance to rotation. For ethane, only 12.6 kJ/mol =>

Propane Conformers Note slight increase in torsional strain due to the more bulky methyl group.

Butane Conformers C2-C3 Highest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees => totally eclipsed (methyl groups)

Butane Conformers (2) Lowest energy has methyl groups anti. Dihedral angle = 180 degrees => Staggered-anti

Butane Conformers (3) Methyl groups eclipsed with hydrogens Higher energy than staggered conformer Dihedral angle = 120 degrees => Eclipsed (hydrogen and methyl)

Butane Conformers (4) Gauche, staggered conformer Methyls closer than in anti conformer Dihedral angle = 60 degrees => Staggered-gauche

Conformational Analysis

Cycloalkanes Rings of carbon atoms (-CH2- groups) Formula: CnH2n Nonpolar, insoluble in water Compact shape Melting and boiling points similar to branched alkanes with same number of carbons Slightly unsaturated compared to alkanes

Naming Cycloalkanes Count the number of carbons in the cycle If the bonds are single then use the suffix “ane” First substituent in alphabet gets lowest number. May be cycloalkyl attachment to chain.

Cis-Trans Isomerism (a type of stereoisomerism) Cis: like groups on same side of ring Trans: like groups on opposite sides of ring

Cycloalkane Stability 6-membered rings most stable Bond angle closest to 109.5 Angle (Baeyer) strain Measured by heats of combustion per -CH2 -

Heats of Combustion/CH2 Alkane + O2  CO2 + H2O 697.1 686.1 664.0 663.6 kJ/mol 658.6 kJ 662.4 658.6 Long-chain

Cyclopropane Large ring strain due to angle compression Very reactive, weak bonds =>

Cyclopropane (2) Torsional strain because of eclipsed hydrogens

Cyclobutane Angle strain due to compression Torsional strain partially relieved by ring puckering =>

Cyclopentane If planar, angles would be 108, but all hydrogens would be eclipsed. Puckered conformer reduces torsional strain.

Cyclohexane Combustion data shows it’s unstrained. Angles would be 120, if planar. The chair conformer has 109.5 bond angles and all hydrogens are staggered. No angle strain and no torsional strain.

Chair Conformer

Boat Conformer

Conformational Energy

Axial and Equatorial Positions

Monosubstituted Cyclohexanes

1,3-Diaxial Interactions

Disubstituted Cyclohexanes

Cis-Trans Isomers Bonds that are cis, alternate axial-equatorial around the ring. => One axial, one equatorial

Bulky Groups Groups like t-butyl cause a large energy difference between the axial and equatoria l conformer. Most stable conformer puts t-butyl equatorial regardless of other substituents. =>

End of Chapter 2