Organic Chemistry

Organic chemistry is the study of carbon compounds.

There are millions of carbon compounds.

Carbon atoms bond to one another to a far greater extent than do atoms of any other element.

Carbon atoms may form rings and chains.

There are three features that are common to organic compounds:

(1) Organic compounds are ordinarily molecular rather than ionic.

(2) Each carbon atom forms a total of four covalent bonds (octet rule).

The bonds may be single (one electron pair), double (two electron pairs), or triple (three electron pairs).

(3) Carbon atoms may be bonded to each other or to other nonmetal atoms, most often hydrogen (-H), a halogen (-X), oxygen (-O- or =), or nitrogen ( – N -, = N -, ≡ N).

We represent organic molecules with structural formulas, which show all bonds present.

Examples: ethanol ethane

You can also write condensed structural formulas.

Examples: C2H5OH CH3CH3

Saturated Hydrocarbons

Saturated hydrocarbons or alkanes are a simple class of hydrocarbons in which all of the carbon – carbons bonds are single bonds.

Saturated means that the molecule contains the maximum number of hydrogen atoms.

Alkane General Formula: CnH2n+2

Examples:

The four bonds involve sp3 hybrid orbitals. The bond angles are 109.50 (regular tetrahedron). This means that propane and higher alkanes arrange in a “zig-zag” fashion.

Compounds having the same molecular formula but different structural formulas are called structural isomers.

Structural isomers are distinct compounds with their own unique chemical and physical properties.

For example, long straight chain isomers have higher boiling points than branched isomers.

Examples:

ExampleDraw structures for the isomers of C5H12.

Nomenclature

The IUPAC devised a system that could be used to name organic compounds.

For straight-chain alkanes the IUPAC name is a single word.

When an alkane features a branched chain, the name consists of two parts:

(1) a suffix that identifies the parent (longest continuous chain) straight chain alkane

(2) a prefix that identifies the branching alkyl group and indicates by number the carbon atom where branching occurs

If the same alkyl group is at multiple branches, the prefixes di-, tri-, etc. are used.

The number in the name must be as small as possible.

Examples:

ExampleAssign IUPAC names to the following:

(a)

(b)

Cycloalkanes are saturated hydrocarbons containing only single bonds.

Cycloalkanes have 2 fewer hydrogen atoms than the corresponding alkane.

Examples: C5H12 (pentane) and C5H10

(cyclopentane)

Common alkanes include natural gas (mostly methane), bottled gas (propane with butane), gasoline (C5to C12).

Unsaturated Hydrocarbons: Alkenes and Alkynes

Unsaturated hydrocarbons contain at least one carbon-carbon multiple bond.

Alkenes contain one carbon-carbon double bond.

C = C

Alkene General Formula: CnH2n

The simplest alkene is ethene.

The molecule is planar and the bond angle is 1200, corresponding to the sp2 hybridization about each carbon.

The double bond consists of one pi and one sigma bond.

The names of alkenes are derived from the corresponding alkane, with two modifications:

(1) the ending is –ene

(2) when necessary a number (as small as possible) is used to designate the double-bonded carbon

Alkynes contain one carbon-carbon triple bond.

C ≡ C

Alkyne General Formula: CnH2n-2

The alkyne names correspond to the alkene names but the ending is changed to –yne.

Examples:

H – C ≡ C – H (ethyne)

CH3 – C ≡ C – CH3 (2-butyne)

Acetylene is linear with a bond angle of 1800.

The triple bond in alkynes consists of a sigma bond and two pi bonds. Each carbon atom is sp-hybridized.

Example What is the molecular formula of (a) the alkane, alkene, and alkyne containing six carbon atoms? (b) the alkane containing ten hydrogen atoms?

ExampleGive the structural formula for the alkene with the IUPAC name 3-ethyl-2-pentene.

Aromatic Hydrocarbons and Their Derivatives

Aromatic hydrocarbons (arenes) are derived from benzene, C6H6.

delocalized pi e-

In line angle (bond-line) formulas it is understood that a carbon atom is at each vertex of the hexagon; hydrogen atoms are not shown.

The hybridization of each carbon is sp2. The molecule is planar with bond angles of 1200.

Benzene was a common solvent but is not used commonly now because it is toxic.

Derivatives of Benzene

Rules for naming arenes:

(1) If there is only one substituent group on the ring, no number is needed since it will always be on the first carbon atom of the ring.

(2) Some monosubstituted benzenes are named by adding the name of the substituted group as a prefix to the word, benzene. These follow the IUPAC rules for naming.

Chlorobenzene

Nitrobenzene

(3) Other substituted benzenes are exceptions to the above IUPAC rule since special names are used for them.

NOTE: MEMORIZE the names of these compounds!

Common monosubstituted benzenes:

benzoic acid

aniline

phenol

toluene

(4) When the benzene ring has two substituted groups attached to it, three isomeric forms of the compound are possible. These positional isomers are given the prefixes: ortho- , meta- and para- respectively.

0-dichlorobenzene

m-dichlorobenzene

p-dichlorobenzene

(5) When three or more substituent groups are on a benzene ring, the carbon atoms in the ring may be numbered clockwise or counterclockwise starting with one of the substituted groups. The numbering must be done in the direction which results in the lowest possible numbers for the substituted groups.

1,2,5-trichlorobenzene

2,4,6-trinitrotoluene (TNT)

ExampleName the following derivatives of phenol:

(a) (b)

Another type of aromatic hydrocarbon forms when two or more benzene rings fuse together.

Examples:

C10H8 – naphthalene

Functional Groups

Many organic molecules are considered to be derived from hydrocarbons by substituting a functional group for a hydrogen atom.

The functional group can be a nonmetal atom or a small group of atoms bonded to carbon.

Alcohols and Ethers

Alcohols:

(1) R-OH (R= alkyl group)

(2) named by substituting the suffix –ol for –ane on the corresponding alkane

(3) Examples:

CH3OH = methanol (methyl alcohol)

CH3 – CH2OH = ethanol (ethyl alcohol)

CH3 - CH2 - CH2OH = 1-propanol (propyl alcohol)

Ethers:

(1) R – O – R1 (R and R1 may be the same or different)

(2) Named by listing the two alkyl groups

(3) Examples:

CH3 – O – CH3 dimethyl ether

CH3 – O – C2H5 ethyl methyl ether

Alcohols (H-bonds) usually have boiling points much higher than alkanes of similar molar mass.

Ethers (no H –bonds) have lower boiling points than alcohols.

Alcohols and ethers with low molar masses are soluble in water.

Denatured ethanol contains small quantities of methanol or benzene.

Some alcohols contain 2 or more –OH groups per molecule. Example: ethylene glycol

A common ether used in lab is diethyl ether, commonly called “ether”.

Aldehydes and Ketones

Aldehydes and ketones contain the carbonyl group: C = O

The simplest ketone is acetone (IUPAC name propanone), a common industrial solvent:

The bond angle about the central atom is 1200.

The simplest aldehyde is formaldehyde (IUPAC name methanal), a carcinogenic preservative:

Higher “aromatic” aldehydes are common as flavorings and perfumes:

vanillin

Example

Classify each of the following as an alcohol, ether, aldehyde, or ketone.

(a) CH3 – O – CH2CH2CH3

H H

(b)CH3 – C – C = 0

OH

(c)

Carboxylic Acids and Esters

The general structure of a carboxylic acid is:

They act as weak acids in water. The acidic hydrogen is the one bonded to oxygen.

The IUPAC name (which is rare) of a carboxylic acid can be obtained by substituting the suffix “–oic acid” for the “e” in the name of the alkane.

The two simplest examples are:

methanoic acid (formic acid)

ethanoic acid (acetic acid)

Treatment of a carboxylic acid with the strong base NaOH forms the sodium salt of the acid.

Example: soaps – sodium salt of long-chain carboxylic acids like stearic acid

The reaction between an alcohol and a carboxylic acid forms an ester:

The common name of an ester has two parts: (1) the first (methyl, ethyl, etc.) is derived from the alcohol (2) the second (formate, acetate, etc.) is the name of the acid with the –ic suffix changes to –ate.

Example: ethyl alcohol + formic acid = ethyl formate.

Many esters have pleasant odors and are commonly found in natural and synthetic fragrances.

Examples: methyl butyrate (apple flavor/odor), octyl acetate (orange), isopentyl acetate (banana)

Example

Show the structure of (a) the three-carbon alcohol with an –OH group at the end of the chain (b) the three-carbon carboxylic acid (c) the ester formed when these two compounds react.

Amines

R may be an alkyl or aromatic group. Primary, secondary, or tertiary amines are possible.

Examples:

methylamine (flammable gas, low boiling pt, unpleasant “fishy smell”, weak base)

aniline (used in making polyurethane)

Amino acids contain both NH2 and COOH groups.

Isomerism

Isomers are distinctly different compounds, with different properties, that have the same molecular formula.

Structural Isomers – the order in which the atoms are bonded differs.

Three structural isomers for C3H8O:

Structural isomers for C4H8:

Example

Consider the molecule C3H6Cl2, which is derived from propane, C3H8, by substituting two Cl atoms for H atoms. Draw the structural isomers for C3H6Cl.

Geometric (cis-trans) Isomers

Geometric isomerism is common among alkenes. It occurs when both of the double bonded carbon atoms are joined to two different atoms or groups.

There is no free rotation about the C=C.

cis – the two groups or atoms are as close as possible

trans – the two identical groups or atoms are farther apart

cis-2-butene

trans-2-butene

Cis and trans isomers differ in physical and (to a lesser degree) chemical properties.

Example

Draw all of the isomers of the molecule C2H2Cl2 in which two of the H atoms of ethylene are replaced by Cl atoms.

Optical Isomers

Optical isomers arise when at least one carbon atom (chiral center) in a molecule is bonded to four different atoms or groups.

Optical isomers are essentially mirror images of each other. It is not possible to place one molecule over another so that identical groups are touching (like right and left hands).

A molecule which exists in two different forms that are not superimposable mirror images is said to be chiral.

The two different forms are called enantiomers or optical isomers.

Molecules with more than one chiral center can have more than two enantiomers.

Example

In the following structural formulas, locate each chiral carbon atom.

The term “optical isomer” comes from the effect that enantiomers have on plane-polarized light.

When light is passed through a solution containing a single enantiomer, the plane is rotated from its original position. One isomer rotates it to the left and the other to the right.

If both isomers are present in the same amounts, we have a racemic mixture and the two rotations offset each other producing no effect on the plane-polarized light.

Poalrimeter

Enantiomers have the same physical properties but different chemical properties (due to different three-dimensional structures).

Chiral drugs are used to treat a wide range of illnesses. About half of the 100 best selling drugs on the market today consist of a single enantiomer.

Enantiomers can be separated by liquid chromatography.

Some enantiomers are prepared by asymmetric synthesis, which uses an enzyme to convert an optically inactive precursor to the drug desired.

Organic Reactions

Characteristics of Organic Reactions:

(1) most species are molecules/pure substances(2) nonpolar solvents

(3) slower than inorganic

There are 4 types of organic reactions:

(1) Addition Reactions – a small molecule (H2, Cl2, HCl, H2O) adds across a double or triple bond.

The above is an example of hydrogenation, effective in converting liquid to solid fats.

(2) Elimination Reactions – is the reverse of an addition reaction. It involves the elimination of 2 groups from adjacent C-atoms, converting a saturated molecule to an unsaturated one.

Example: dehydration of ethanol

(3) Condensation Reactions – similar to

elimination, occurs when molecules combine by splitting out a small molecule such as H2O.

Example: esterification

(4) Substitution Reactions – a reaction where an atom or group of atoms in a molecule is replaced by a different atom or group.

Examples:

Chlorination of alkane/benzene

CH4(g) + Cl2(g) --- CH3Cl(g) + HCl(g)

C6H6(l) + Cl2(g) ----FeCl2 --- C6H5(l) + HCl(g)

Nitration of benzene

C6H6(l) + HNO3(l) ----H2SO4 --- C6H5NO2(l) + H2O(l)

Formation of alcohol from alkyl halide

C2H5Br(l) + OH-(aq) --- C2H5OH(aq) + Br-

(aq)

Example

Classify each of the following as an addition, elimination, condensation, or substitution reaction.

(a) C2H5OH(aq) + H+(aq) + Br-

(aq) - C2H5Br(l) + H2O(l)

(b) C6H12(l) + Cl2(g) -- C6H12Cl2(l)

(c) C7H8(l) + 2 Cl2(g) - C7H6Cl2(l) + 2 HCl(g)

(d) C5H11(l) -- C5H10(l) + HCl(g)