Ch. 22: Organic Chemistry• Organic chemistry is the branch of chemistry that studies carbon compounds.

• Organic compounds were originally thought to be synthesized only in living organisms until…

- Friedrich Wöhler synthesized urea (1828) to dispel the idea.

• Now many organic compounds are synthesized: synthetic oil, gasoline, rubber, plastics…

General Characteristics of Organic Compounds• Structure: Carbon can exhibit three different types of hybridization…

- sp3 hybridized carbons for tetrahedral geometries

- sp2 hybridized carbons for trigonal planar geometries

- sp hybridized carbons for linear geometries

Carbon’s Hybridization

Remember: Carbon’s hybridization relates to the # of atoms that it is bonded to… 4 atoms= sp3 3 atoms= sp2 2 atoms= sp

The Stabilities of Organic Molecules• Stability: Carbon also forms strong bonds with itself.

• Therefore, C can form stable long chain or ring structures.

cyclopropane

(unstable)

CnH(2n+2) CnH(2n)

The Stabilities of Organic Molecules• Remember:

- Bond strength increases from single to double to triple bond.

- Bond length decreases in the same direction.

- Strongest & shortest bond C ≡ C

• Carbon and hydrogen have very similar electronegativities, so the C-H bond is essentially non-polar.

• Therefore, compounds containing C-C and C-H bonds are stable.

• Adding certain groups of atoms called “functional groups” introduces reactivity into organic molecules.

Solubility of Organic Compounds• Solubility in water varies:

• Long nonpolar molecules are insoluble in water.

• Polar groups that are added will increase solubility.

Introduction to Organic Compounds• The simplest type of organic compounds are the “alkanes”.

• Alkanes contain only single bonds.

• These compounds are referred to as “saturated hydrocarbons”…C is allowed to bond to the maximum number of hydrogen atoms.• Alkanes have the general formula: CnH(2n+2)

Isomers• “Straight chain” hydrocarbons have each C atom joined in a continuous chain.

• In a straight chain hydrocarbon no one C atom may be attached to more than two other C atoms.

• Straight chain hydrocarbons are not linear!

• Each C atom is tetrahedral, so the chains are actually bent.

• Branched chain hydrocarbons are possible for four or more C atoms, which give rise to “isomers”…same formula, different properties

• Structural isomers are compounds having the same molecular formula but differ in the bonding arrangement of the atoms.

- Example: There are 2 hydrocarbons with the formula C4H10.

Structural Isomers

Structural IsomersPentane (C5H12) has 3 structural isomers.

• The # of isomers greatly increases with the # of carbon atoms:

Octane, (C8H18), has 18 isomers and decane, (C10H22), has 75 isomers!

Stereoisomers• Stereoisomers have the same formula and the same bonding of atoms, but they differ in the spatial arrangement of the atoms.

- Example: “Geometric Isomers”

Since the double bond cannot rotate, they are truly different!

Stereoisomers• Optical isomers (or enantomers) are also stereoisomers. They are molecules that are “nonsuperimposable mirror images” of each other.

• They rotate plane polarized light differently.

• In order to have an optical isomer, you must have a “chiral center”…a carbon atom with 4 different attached groups.

Optical Isomer

2-bromopentane

Optical Isomer

Enantiomers of alanine, an amino acid

Optical Isomers• Here’s another example of optical isomers…

• Consider the compound carvone:

- The one on the left is responsible for the odor and taste of caraway and dill.

- The one on the right is isolated from spearmint.

Unsaturated Hydrocarbons• Alkenes: Contain a double bond…C = C.

• The double bond increases its reactivity. (Often times, elements are added to the compound via the double bond being broken.)

• The simplest alkenes are H2C=CH2 (ethene) and CH3CH=CH2 (propene)…(their trivial names are ethylene and propylene.)• Alkenes are named in the same way as alkanes with the suffix -ene replacing the -ane in alkanes.• The location of the double bond is indicated by a number.

(We will get more into naming compounds later!)

Alkenes

Geometric isomers (“cis-trans”) only occur in compounds with a double bond.

Unsaturated Hydrocarbons• Alkynes: Contain a triple bond…C ≡ C.

• The triple bond increases its reactivity even more vs. C = C.

• The simplest alkyne is HC≡CH (ethyne, or acetylene.)

• When it is burned in the presence of oxygen (oxyacetylene torch) the temperature is about 5300º F!• Alkynes are named in the same way as alkenes with the suffix -yne replacing the -ene for alkenes.

Unsaturated Hydrocarbons• Aromatic hydrocarbons: carbon atoms are connected in a planar ring structure. (They have a distinct odor to them.)

- The carbons are linked by both σ and -bonds.

- The best known example is benzene (C6H6).

Aromatic HydrocarbonsBenzene (C6H6).

The delocalized -bonds makes aromatic compounds rather stable.

Summary of Hydrocarbons

Naming Organic Compounds• All organic molecule names have three parts:

- Prefix, which tells the nature of the substituents.

- Base, which gives the number of carbons.

- Suffix, which gives the family (alkanes, alkenes, etc.)

• Rules for naming compounds are given by the International Union for Pure and Applied Chemistry (IUPAC).

• We will only do one example in these notes for practice, but we will have more to do together later on!

• Name this organic compound:

• Step 1: Find the longest chain and use it as the base name of the compound.

Naming Organic Compounds

heptane

7 C’s

• Step 2: Number the carbon atoms starting with the end closest to the substituents.

- Groups attached to the main chain are called substituents.

- The preferred numbering will give substituents the lowest numbers.

Naming Organic Compounds

1 3

5 6 74

2

2- ________ 4- _________heptane

• Step 3: Name and give the location of each substituent.

- When two or more substituents are present list them in alphabetical order!

- A substituent group formed by removing an H atom from an alkane is called an alkyl group.

• Alkyl groups are named by replacing the –ane ending with -yl.

Examples: CH4 is methane, and a –CH3 group is a methyl group.

–CH2CH3 would be an ethyl group.

–CH2CH2CH3 would be a propyl group.

• When there are two or more of the same substituent, the number of that type of substituent is indicated by a prefix…

Examples: “dimethyl” indicates two methyl group substituents.

“dichloro” indicates two chlorine substituents.

Naming Organic Compounds

• Let’s name the substituents…

Naming Organic Compounds

1 3

5 6 74

2

methyl-

ethyl-

Put them in alphabetical order!!!

4-ethyl-2-methylheptane

Functional Groups• Hydrocarbons are relatively unreactive. For an organic molecule to be reactive it needs something additional…

• A site of “reactivity” in an organic molecule is called a functional group. Functional groups control how a molecule functions.

• These groups determine the chemistry of the molecule.

• Functional group containing molecules can either be saturated… (single bonds only-- alcohols, ethers, amines, etc.) or unsaturated… (double bonds involved--carboxylic acids, esters, amides, etc.)

(We usually use R to represent alkyl groups… methyl, ethyl, etc.)

Functional Groups

Functional Groups

Properties of Functional GroupsAlcohols (R-OH): do not liberate OH− ions in solution like bases

Examples: CH3OH, methanol, is used as a gasoline additive and a fuel.

CH3CH2OH, ethyl alcohol, or ethanol, is “booze”.

Properties of Functional Groups

R

O

H

Ethers (R-O-R′): used as anesthetics, solvent for gums, fats, and wax.

Example: diethyl ether (CH3CH2 – O – CH2CH3)

Aldehydes: Example: methanal (or formaldehyde) (CH2=O) is used as a preservative for biological specimens…it coagulates proteins.

Ketones: Examples: fingernail polish remover is propanone [or acetone… (CH3)2C=O ] used as solvents for plastics;

manufacture of cordite, an explosive; carvone, (the optical isomer example) is a ketone.

R

O

R'

Properties of Functional Groups

R

O

OH

Carboxylic Acids (COOH): found in fruits… (ascorbic acid, or vitamin C; malic acid is in apples); Acetic acid (CH3COOH) is vinegar; Formic acid (HCOOH) was first obtained by distillation of red ants (responsible for the sting in ant bites.)

Carboxylic acids are very weak acids/weak electrolytes.

Properties of Functional GroupsEsters (RCOOR’): often have distinctive odors: ethyl

butyrate (C3H7COOC2H5) has the odor of pineapple; butyl acetate (CH3COOC4H9) has the odor of bananas.

Other Examples:

benzocaine (in sun burn lotions);

ethyl acetate (nail polish remover);

vegetable oils;

polyester thread;

aspirin…which has an ester functional group as well as a carboxylic acid functional group (see previous slide)

R

O

OR'

Properties of Functional GroupsAmines and Amides (R3N):

• Amines are organic bases where R may be H or a hydrocarbon group.

• Just as alcohols, (R – OH), can be thought of organic forms of water, (H – OH) amines can be thought of organic forms of ammonia, NH3.

Example: ethylamine, (CH3CH2NH2)

• Amines are among the most noxious-smelling of all organic compounds.

• Amides are composites of carbonyl and amine functional groups:

•Amides are the key functional group for amino acids which link up to make proteins.

RC

N

O

R'

H

amine groupcarbonyl group

Functional Groups

Practice Problem:

What functional groups do you see highlighted in these sugars?

Answers:

* Glucose is an aldehyde.

* Fructose is a ketone.

* Both contain multiple hydroxyl (or alcohol) functional groups too.