Tizazu

Alkanes, Alkenes, Alkynes and Benzene

Organic compounds: organic chemistry is the chemistry of carbon and only a few other

elements-chiefly, hydrogen, oxygen, nitrogen, sulfur, halogens, and phosphorus (from 116

elements).

Note: In the early days of chemistry, scientists though organic compounds were those

produced by living organisms and they could not synthesize any organic compound by

starting with only inorganic compounds. In 1828, Friedrich Wöhler synthesized the first

organic compound in his laboratory (urea).

Today, chemists obtain organic compounds in two principal ways: 1.Isolation from nature and

2. Synthesis in the laboratory.

Note: Compounds made in the laboratory are identical in both chemical and physical

properties to those found in nature.

Why organic chemistry: we can find organic compounds everywhere around us (foods,

flavors, fragrances, medicines, toiletries, cosmetics, plastic, paints, our body, and etc.).

Chemist have discovered or synthesized more than 10 million of organic compounds.

However, 1.7 million inorganic compounds are discovered or synthesized (85% of all known

compounds are organic compounds).

Typical properties of organic compounds: 1. They contain carbon atom. 2. Bonding is

almost entirely covalent (covalent compounds). 3. They have low boiling points and low

melting points. 4. They are flammable (almost all burn). 5. They are soluble in nonpolar

compounds and most are insoluble in water. 6. Many are gases, liquids, or solids.

It is important to know:

•Carbon • normally forms four covalent bonds and has no unshared pairs of electrons.

•Hydrogen: • forms one covalent bond and has no unshared pairs of electrons.

•Nitrogen: normally forms three covalent bonds and has one unshared pair of electrons.

•Oxygen: normally forms two covalent bonds and has two unshared pairs of electrons.

Halogen: normally forms one covalent bond and has three unshared pairs of electrons.

Functional group: an atom or group of atoms within a molecule that shows a characteristic

set of predictable physical and chemical behaviors.

Functional groups are important in organic chemistry because:

NH4Cl AgNCO H2N-C-NH2

+ AgCl

heat

Ammonium

chloride

Silver

cyanate

Urea Silver

chlorideDr. Behrang Madani Chemistry B11 Bakersfield college

1. They are sites predictable chemical behavior. A particular functional group, in whatever

compound it is found, undergo the same types of chemical reactions.

2. Determine in large measure the physical properties of a molecule.

3. Serve as the units by which we classify organic compounds into families.

4. Serve as a basis for naming organic compounds.

Hydrocarbons: a large family of organic compounds and they contain only carbon and

hydrogen.

Hydrocarbons are divided into two groups:

1. Saturated hydrocarbon: a hydrocarbon that contains only carbon-carbon single bonds

(alkanes, also called Aliphatic hydrocarbons). Saturated in this context means that each

carbon in the hydrocarbon has the maximum number of hydrogen atoms bonded to it.

2. Unsaturated hydrocarbon: a hydrocarbon that contains one or more carbon-carbon

double bonds, triple bonds, or benzene rings.

Alkanes: They are saturated hydrocarbons (they have only carbon-carbon single bonds). The

molecular formula of this group is CnH2n+2 (n is the number of carbon atoms).

Naming the unbranched alkanes: Chemists have adopted a set of rules established by the

International Union of Pure and Applied Chemistry (IUPAC). The IUPAC name for an alkane

consists of two parts: 1. A prefix that shows the number of carbon atoms (meth-, eth-, prop-,

but-, pent-, hex-, hept-, oct-, non- and dec-). 2. The suffix “-ane”.

CH4 Methane C2H6 Ethane C3H8 Propane C4H10 Butane

Note: we can represent the formula of an organic compound by the molecular formula or by

the structural formula. Structural formulas can be represented by three ways: Expanded

(Complete) structural formula, Condense structural formula, and Line-angle formula:

Expanded (complete) structural formula: to represent this model, the carbon atom is shown

attached to the hydrogen atoms (we show all connections). Dr. Behrang Madani Chemistry B11 Bakersfield college

Condensed structural formula: to represent this model, the hydrogen atoms are grouped

with their carbon atom. The number of hydrogen atoms is written as subscript.

CH3-CH2-CH2-CH2-CH3 or CH3-(CH2)3-CH3

Line-angle formula: is a form of the structural formula. A line represents a carbon-carbon

bond and a vertex represents a carbon atom. A line ending in space represents a –CH3 group.

CH3-CH2-CH3 Propane

CH3-CH2-CH2-CH2-CH3 Pentane

Substituent groups: they are the branches in organic compounds. A substituent group

derived from an alkane by removal of a hydrogen atom is called an alkyl group (R-). Alkyl

groups are named by dropping the “-ane” from the name of the parent alkane and adding the

suffix “-yl”.

CH3- Methyl C2H5- Ethyl C3H7- Propyl

Note: some substituents derived from other elements or other spices than alkanes:

-F Fluoro -Cl Chloro -OH Hydroxyl -NO2 Nitro

Naming branched alkanes: 1. Write the alkane name of the longest continuous chain of

carbon atoms (parent chain or root chain). 2. Number carbon atoms starting from the end

nearest substituent. 3. Give the location and name of each substituent (alphabetical order) as a

prefix to the alkane name (main chain). Use a hyphen to connect the number to the name.

CH3-CH-CH2-CH3

In this example, the longest chain is Butane. We number carbon atoms starting from the end

nearest substituent (left to right). The location of subtituent is 2 and its name is methyl.

Therefore, the complete name of this compound is 2-Methylbutane.

CH3-CH2-CH-CH-CH3 3-Chloro-2-methylpentane

Constitutional isomers: compounds with the same molecular formula but a different

connectivity of their atoms (different structural formulas).

Butane: C4H10 CH3-CH2-CH2-CH3 Methylpropane: C4H10 CH3-CH-CH3

Note: Constitutional isomers are different compounds and have different physical and

chemical properties.

CH3

1 2 3 4

Cl CH3

5 4 3 2 1

CH3Dr. Behrang Madani Chemistry B11 Bakersfield college

Cycloalkane: a saturated hydrocarbon that contains carbon atoms bonded to form a ring. A

hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon.

Physical properties of alkanes: 1. They are nonpolar compounds (the electronegativity

difference between carbon and hydrogen is 2.5-2.1 = 0.4). 2. The only interactions between

their molecules are the very week London dispersion forces. 3. They are insoluble in water

(because water is polar) and they are soluble in nonpolar organic compounds. 4. They have

the lower density than water (their densities is between 0.7 and 0.8 g/mL). 5. They have the

low boiling points and the low melting points. 6. They can be gases (with 1 to 4 carbon

atoms), liquids (with 5 to 17 carbon atoms), or solids (with 18 or more carbon atoms).

Note: In general, both boiling and melting points of alkanes increase with increasing

molecular weight (the number of carbon atoms).

Note: In general, both boiling and melting points of alkanes decrease with increasing the

number of branches (for alkanes with the same molecular weights). As branching increases,

the alkane molecule becomes more compact and its surface area decreases (London dispersion

forces act over a smaller surface area).

Chemical properties of alkanes: in general, they have a low reactivity (inert). Their most

important reactions are combustion (reaction with oxygen) and halogenation (reaction with

halogens).

Combustion: alkanes react with oxygen (they are oxidized). In this reaction, CO2, H2O, and

energy (heat) are produced.

CH4 + 2O2 → CO2 + 2H2O + energy (heat)

Halogenation: alkanes react with chlorine and bromine if we heat the mixture or if we expose

the mixture to light (in the dark at room temperature, nothing happens).

CH4 + Cl2 CH3Cl + HCl

If chlromethane is allowed to react with more chlorine:

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

Cyclobutane Cyclopentane

heat or light

Chloromethane

Dichloromethane

Trichloromethane

heat or light

heat or lightDr. Behrang Madani Chemistry B11 Bakersfield college

CHCl3 + Cl2 CCl4 + HCl

Sources of alkanes: the two major sources of alkanes are natural gas and petroleum. Natural

gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other

relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick,

viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed the

decomposition of marine plants and animals. The fundamental separation process in refining

petroleum is fractional distillation. All crude petroleum that enters a refinery goes to

distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into

fractions.

Alkene: an unsaturated hydrocarbon that contains one or more carbon-carbon double bonds.

The molecular formula of this group is CnH2n (n is the number of carbon atoms). Alkenes

have less hydrogen atoms than alkanes.

C2H4 CH2=CH2 C3H6 CH2=CH-CH3

Alkynes: an unsaturated hydrocarbon that contains one or more carbon-carbon triple bonds.

The molecular formula of this group is CnH2n-2 (n is the number of carbon atoms). Alkynes

have less hydrogen atoms than alkanes and alkenes.

C2H2 CH CH C3H4 CH C-CH3

Naming unbranched alkenes and alkynes: we use the IUPAC system of naming for

alkanes. For alkenes, we replace the suffix “-ane” of alkanes by “-ene”. For alkynes, we

replace the suffix “-ane” of alkanes by “-yne”.

Note: Some alkenes and alkynes (particularly those of low molecular weight) are known

almost exclusively by their common names (we show them here in the parenthesis).

Naming branched alkenes and alkynes: 1. Name the longest carbon chain that contains the

double or triple bond. 2. Number the carbon chain starting from the end nearest the double or

triple bond. 3. Give the location and name of each substituent (alphabetical order) as a prefix

to the alkene or alkyne name.

CH3-CH=CH-CH3 2-butene

C C-CH2-CH3 1-butyne

CH3-CH2-C=CH-CH3 3-ethyl-2-pentene

CH2-CH3

Tetrachloromethane

heat or lightDr. Behrang Madani Chemistry B11 Bakersfield college

Cis and trans isomers: isomers that have the same connectivity of their atoms (and the same

molecular formulas) but a different arrangement of their atoms in space. Specifically, cis and

trans stereoisomers result from the presence of either a ring or a carbon-carbon double bond

(not a carbon-carbon triple bond). Cis and trans isomers are different compounds and have

different physical and chemical properties.

C = C C = C

trans-2-Butene cis-2-Butene

Note: Both boiling and melting points of the cis isomers are lower than the trans isomers

(because the surface areas of cis isomers are smaller and molecules are more compact).

Physical properties of alkenes and alkynes: their physical properties are similar to those of

alkanes with the same carbon skeletons.

Chemical properties of alkenes and alkynes: these organic compounds are more reactive

than alkanes. The most characteristic reaction of alkenes (alkynes) is addition to the carboncarbon double bond (triple bond): The double bond is broken and in its place single bonds

form to two new atoms or groups of atoms. We can name four important chemical reactions

for them: 1. Addition of hydrogen (Hydrogenation or Reduction). 2. Addition of hydrogen

halides (Hydrohalogenation). 3. Addition of water (Hydration). 4. Addition of bromine and

chlorine (Halogenation).

Hydrogenation or Reduction (addition of hydrogen): atoms of hydrogen add to the carbons

in a double or triple bond to form alkanes. A catalyst as platinum (Pt), nickel (Ni), or

palladium (Pd) is added to speed up the reaction. The transition metal catalysts used in this

hydrogenation are able to absorb large quantities of hydrogen onto their surfaces, probably by

forming metal-hydrogen bonds.

HC = CH + H2 HC - CH

Halogenation (addition of chlorine and bromine): chlorine and bromine react with alkenes

(alkynes) at room temperature by addition of halogen atoms to the carbon atoms of the double

bond (triple bond). We do not need any catalysts for this reaction (in general, we use an inert

solvent, such as dichloromethane, CH2CH2).

HC = CH + Cl2 HC - CH

H3C

CH3

H3C CH3

H H H

H H H H

H H

H H H H

Cl Cl

PtDr. Behrang Madani Chemistry B11 Bakersfield college

Note: Addition of bromine is a useful qualitative test for the presence of an alkene (or an

alkyne). If we dissolve bromine in carbon tetrachloride, the solution is red. In contrast,

alkenes (alkynes) and dibromoalkanes are colorless. The disappearance of the red color as

bromine adds to the double bond (triple bond) tells us that alkene (alkyne) is, indeed present.

Note: addition of halogen to an alkene (an alkyne) is an addition reaction and two atoms of

halogens are added to the carbon atoms of the double bond (triple bond). However, addition

of halogen to an alkane is a substitution reaction and only one atom of halogen is replaced

by one hydrogen atom.

Benzene: a molecule of benzene consists of a ring of six carbon atoms with onr hydrogen

atom attached to each carbon. It has the molecular formula C6H6. The real molecule of

benzene is a resonance hybrid of the two Lewis structures (a unique feature that makes

benzene chemically stable).

Aromatic compounds: they are unsaturated hydrocarbons that contain one or more benzene

rings in their structures.

Arene: a compound containing one or more benzene-like rings.

Naming aromatic compounds:

1. When one substituent group is attached to benzene, we write the name of the group in front

of benzene.

Note: The IUPAC system retains certain common names for several of the simpler

monosubstituted alkylbenzenes:

2. When two or more substituents are attached to benzene, the ring is numbered give the

lowest numbers to the substituents. The substituents are listed alphabetically.

Cl +

Chlorobenzene

NO2 +

Nitrobenzene

CH2CH3

Ethylbenzene

OCH3 C-OH

OH NH2 C-H

Phenol Aniline Benzoic acid Anisole Benzaldehyde

CH3

Toluene

C C

H H

HDr. Behrang Madani Chemistry B11 Bakersfield college

Note: if we have two substituents, we can also use the prefix “ortho-”, “meta-”, and “para-”

to show the position of the substituents (these names are common names).

Note: The susbtituent group derived by loss of an H from benzene is called phenyl group,

C6H5-.

Chemical properties of benzene and its derivatives: The most important characteristic

reaction of aromatic compounds is substitution at a ring carbon (aromatic substitution). We

can name for them these three reactions: 1.Halogenation 2. Nitration 3. Sulfonation.

Halogenation: in the present of an iron catalyst, chlorine reacts rapidly with benzene to give

chlorobenzene and HCl.

Nitration: when we heat benzene or one of its derivatives with a mixture of concentrated

nitric and sulphuric acids, a nitro (-NO2) group replaces one of the hydrogen atoms bonded to

the ring. In this reaction, sulfuric acid is a catalyst and it is added to speed up the reaction.

COOH

CH3

CH2CH3

2-Bromobenzoic acid

(o-Bromobenzoic acid)

1,3-Dimethylbenzene

(m-Xylene)

1-Chloro-4-ethylbenzene

(p-Chloroethylbenzene)

3 2

H Cl

FeCl

+ Cl HCl +

Benzene Chlorobenzene

H HNO3

H2SO4

+ NO2 + H2O

Nitrobenzene