CH 3: Organic Compounds: Alkanes and Their Stereochemistry

Transcript CH 3: Organic Compounds: Alkanes and Their Stereochemistry

CH 3: Organic Compounds: Alkanes and Their Stereochemistry

Renee Y. Becker CHM 2210 Valencia Community College 1

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 2

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 3

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  &  bonds?

Functional Groups with Carbon Singly Bonded to an Electronegative Atom 5

Groups with a Carbon –Oxygen Double Bond (Carbonyl Groups) 6

Alkanes •

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 C n H 2n+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

Naming Alkanes Memorize 1-10 for next class 13

Alkane Isomers • The molecular formula of an alkane with more than three carbons can give more than one structure – C 4 (butane) = butane and isobutane – C 5 (pentane) = pentane, 2-methylbutane, and 2,2 dimethylpropane • Alkanes with C’s connected to no more than 2 other C’s are

straight-chain

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 15

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 – CH 3 CH 2 CH 2 CH 3 – CH 3 (CH 2 ) 2 CH 3 (butane) (butane) 16

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 ending of alkane with -

– CH 3 is “methyl” (from methane) – CH 2 CH 3 is “ethyl” from ethane 17

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 group) alkyl – a carbon with three carbons attached to it ( tertiary alkyl group) 18

Types of Hydrogens 19

Substituents 20

• Isobutane, “isomer of butane” Common Names • Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain.

• Neopentane, most highly branched • Five possible isomers of hexane, 18 isomers of octane and 75 for decane! 21

Pentanes H H H H H H H H C H C H C H C H C H C H H H H C H C H H C H H C H

-pentane, C 5 H 12 H 3 C CH 3 C CH 3 isopentane, C 5 H 12 CH 3 neopentane, C 5 H 12 H 22

IUPAC Names • Find the longest continuous carbon chain.

• Number the carbons, starting closest to the first branch.

• Name the groups attached to the chain, using the carbon number as the locator.

• Alphabetize substituents.

• Use di-, tri-, etc., for multiples of same substituent.

Longest Chain • The number of carbons in the longest chain determines the base name: ethane, hexane. • If there are two possible chains with the same number of carbons, use the chain with the

most

substituents.

H 3 C CH 2 H 3 C CH CH 2 CH 3 C CH CH 2 CH 3 CH 2 CH 3 CH 3

Number the Carbons • Start at the end closest to the first attached group.

• If two substituents are equidistant, look for the next closest group.

1 H 3 C CH 3 CH 2 3 CH 4 CH 2 CH 2 CH 3 5 CH 2 CH 3 CH 6 CH 3 7 25

• CH 3 -, methyl • CH 3 CH 2 -, ethyl CH 3 CH CH 3 isopropyl CH 3 CH CH 2

sec-

butyl CH 3 Name Alkyl Groups • CH 3 CH 2 CH 2 -,

propyl • CH 3 CH 2 CH 2 CH 2 -,

butyl CH 3 CH 3 CH CH 2 isobutyl H 3 C CH 3 C CH 3

tert

-butyl 26

Propyl Groups H H C H H C H H C H H

-propyl A primary carbon H H C H H C H H C H H isopropyl A secondary carbon 27

Butyl Groups H H C H H C H H C H H C H H

-butyl A primary carbon H H C H H C H H C H H C H H

sec

-butyl A secondary carbon 28

H H H C H H H C C C H H H H isobutyl A primary carbon Isobutyl Groups H H H C H H H C H C H C H H

tert

-butyl A tertiary carbon 29

• Alphabetize substituents by name.

• Ignore di-, tri-, etc. for alphabetizing.

H 3 C CH 3 CH CH CH 2 CH 2 CH 3 CH 2 CH 3 CH CH 3 3-ethyl-2,6-dimethylheptane Alphabetize 30

Write structures for the following: a) 3-ethyl-3-methylpentane b) 4-t-butyl-2-methylheptane c) 5-isopropyl-3,3,4-trimethyloctane d) 3-ethyl-2,4,5-trimethylheptane Example 1 31

Provide IUPAC & common names (1-3) Example 2 32

Name the following Example 3 a) 3,3-dimethyl-4-isobutylheptane b) 3,3-dimethyl-4-tert-butylheptane c) 4-tert-butyl-3,3-dimethylheptane d) 4-isobutyl-3,3-dimethylheptane 33

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.

• For alphabetizing use the first letter of the complex sub. Even if it is a numerical (di, tri, etc) 34

Complex Examples 35

Draw the structures and give their more common names Example 4 a) (1-methylethyl) group b) (2-methylpropyl) group c) (1-methylpropyl) group d) (1,1-dimethylethyl) group 36

Draw the structures a) 4-(1-methylethyl)heptane b) 5-(1,2,2-trimethylpropyl)nonane Example 5 37

Assignment

• For next class draw the 9 isomers of heptane and name them 38

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 • They react with Cl 2 in the presence of light to replace H’s with Cl’s (not controlled) 39

Physical Properties: Alkanes • Solubility: hydrophobic • Density: less than 1 g/mL • Boiling points increase with increasing carbons (little less for branched chains).

• 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.

Branched Alkanes • Lower b.p. with increased branching • Higher m.p. with increased branching CH 3 CH CH 3 CH 2 CH bp 60°C mp -154°C 2 CH 3 CH 3 CH 3 CH 3 CH CH bp 58°C mp -135°C CH 3 C H 3 CH 3 C CH 2 CH 3 CH 3 bp 50°C mp -98°C 43

Example 6 List each set of compounds in order of increasing boiling point and melting point 44

Example 7 Which of the following has the highest boiling point?

a) 3-methylpentane b) 2,2-dimethylbutane c) Hexane d) Methane 45

Conformers

• Conformation

- Different arrangement of atoms resulting from bond rotation • Conformations can be represented in 2 ways: 46

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 47

Conformations of Ethane • Stereochemistry concerned with the 3-D aspects of molecules • Rotation is possible around C-C bonds in open chain molecules (not cyclic) 48

Ethane Conformers • Eclipsed conformer has highest energy • Dihedral angle = 0 degrees 49

Conformational Analysis • Torsional strain: resistance to rotation.

• For ethane, only 3.0 kcal/mol 50

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 52

Butane Conformers (2) • Lowest energy has methyl groups

anti

• Dihedral angle = 180 degrees anti 53

Butane Conformers (3) • Methyl groups eclipsed with hydrogens • Higher energy than staggered conformer • Dihedral angle = 120 degrees eclipsed 54

Butane Conformers (4)

• Gauche

, staggered conformer • Methyls closer than in anti conformer • Dihedral angle = 60 degrees gauche 55

Conformational Analysis 56

• Anti conformation is lowest in energy.

• “Straight chain” actually is zigzag.

Higher Alkanes

CH 3 CH 2 CH 2 CH 2 CH 3 H H C H H H C C C H H C H H H H H