Organic Chemistry

Organic Chemistry Larry Scheffler Lincoln High School Revised September 12, 2010

Orgins • Originally defined as the chemistry of living materials or originating from living sources • Wohler synthesized urea from non organic sources • Now generally defined as the chemistry of carbon and its compounds

Classifying Organic Compounds • Many configurations of carbon atoms are possible in a molecule. • Empirical and molecular formulas are the same for many different compounds. • The relative locations of various combinations of atoms in a formula is very important. • Structural formulas show the location and bonding pattern for each atom in a formula.

Structural Formulas • The two molecules below both have the same empirical and molecular formulas but they have very different characteristics. They are considered different compounds.

Functional Groups

Functional Groups • Many larger molecules have more than one functional group.

Functional Groups • The physical and chemical properties of organic compounds are related to their functional groups. • Compounds may have different numbers of carbon atoms but the same functional group(s) will often have similar properties.

Homologous Series • Compounds that have the same general formula but differing lengths of carbon chains form a homologous series.

Homologous Series: Ex 1 • Each compound in this series differs from the previous compound by a –CH2 – • The general formula for these compounds could be written as CnH2n+2 A homologous series of alkanes

Homologous Series: Ex 2 • Each compound in this series differs from the previous compound by a –CH2 – • The general formula for these compounds could be written as CnH2n A homologous series of alkenes

Homologous Series: Ex 3 • Each compound in this series differs from the previous compound by a –CH2 – • The general formula for these compounds could be written as CnH2n+1OH A homologous series of alcohols

Hydrocarbons • Hydrocarbons are organic compounds that are made up of only carbon and hydrogen. • There are several different categories of hydrocarbons including: • Alkanes • Alkenes • Alkynes • Cyclic Hydrocarbons • Aromatic hydrocarbons

Alkanes • Alkanes are hydrocarbons that have only C-C single bonds. • Examples • Methane CH4 • Ethane CH3-CH3 • Propane CH3-CH2-CH3 • Butane CH3-CH2-CH2-CH3 • Pentane CH3-CH2-CH2-CH2-CH3

Physical Properties of Organic Compounds Boiling points of organic compounds reflects the strength of the intermolecular attractive forces that hold the molecules of a substance together in a condensed phase Organic compounds typically melt and boil at much lower temperatures than most inorganic compounds. The melting and boiling points correlate with the polarity of the molecules and their molecular mass

Physical Properties of Organic Compounds Boiling Point Trends The melting points and boiling points of most organic compounds increase with increasing molecular mass

Physical Properties of Organic Compounds The boiling points of most organic compounds increase with increasing molecular polarity. The above compounds have similar molar masses, but their boiling points vary considerably.

Solubility of Organic Compounds The solubility of organic compounds in water depends on the polarity of any attached functional groups. Neutral organic compounds tend to be hydrophobic, that is, they are less soluble in water than in organic solvents. Some organic compounds that contain polar groups such as low molecular weight alcohols, amines, and carboxylic acids dissolve in water. They are sufficiently polar for hydrogen bonding to occur.

Solubility of Organic Compounds Organic compounds tend to dissolve in a variety of organic solvents. These solvents can be either pure substances like acetone or ethyl alcohol, or mixtures such as petroleum ether. The solubility of organic compounds in these different solvents depends upon the type of solvent and in particular the functional groups if present. As the molar mass of organic compounds increases and the size of the hydrocarbon chain increases the compound becomes more hydrophobic.

Aromatic Structures • The benzene ring is a common structure in organic molecules • It consists of 6 carbon atoms and 6 hydrogen atoms. • One would predict that there should also be 3 C=C bonds in a benzene ring 21

Aromatic Structures • Further investigation reveals that the double bonds are not distinct in benzene. Rather it is a resonance hybrid. • Either of these structures could be used to represent benzene. 22

Aromatic Structures • Research shows that there are no differences in the C to C bonds in benzene. • The current view of benzene holds that there are 6 C-C single bonds and 3 pairs or 6 delocalized electrons 23

Aromatic Structures • The structure of benzene is shown as either of these two structures, or as a circle in a hexagon which depicts that the electrons are delocalized 24

Fused Aromatic Structures • Aromatic hydrocarbons are not limited to a single ring • The fused ring structure shown is Naphthalene 25

Reactions of Hydrocarbons • Hydrocarbons tend to be very unreactive compounds when compared to other organic molecules. • Most hydrocarbons are flammable. They burn in the presence of EXCESS oxygen to form carbon dioxide and water vapor. • Examples: 26

Reactions of Hydrocarbons • Hydrocarbons burn in the LIMITED presence of oxygen to form carbon monoxide* and water vapor. • *carbon monoxide binds to hemoglobin BETTER than oxygen. • Hydrocarbons burn in VERY LIMITED presence of oxygen form carbon and water vapor (soot). 27

Reactions of Hydrocarbons • Most Hydrocarbons undergo substitution reactions in the presence of ultraviolet light • Examples: 28

Reactions of Alkenes • Hydrocarbons that have –C=C- undergo addition reactions. 29

Petroleum • Crude oil is a mixture of hydrocarbons formed over along period of time from the slow decay of plant and animal matter • It is separated by distillation into a variety of fractions 30

Petroleum • Crude 31

Gasoline • Gasoline is a mixture of hydrocarbons. • The grade of a gasoline is based on a system known as an octane rating. • Isooctane is a major component in gasoline that burns evenly. It has octane rating of 100 • Heptane burns with small explosions and tends to cause engines to “knock” • The octane rating is the percentage of isooctane in the gasoline mixture • For an example: Gasoline with an octane rating of 87% isooctane and 13 % heptane. 32

Natural Gas • Natural gas is about 85% methane 33

Halogenoalkanes or Alkyl Halides • Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine). • Halogenoalkanes are commonly known as alkyl halides 34

Halogenoalkanes or Alkyl Halides • Depending on the location of the halogen atom, halogenoalkanes may be primary secondary or tertiary 35

Boiling Points of Halogenoalkanes • The boiling point depends on the halide Cl < Br < I • The boiling points increase as the chain length increases 36

Solubility of Halogenoalkanes • The halogenoalkanes are only very slightly soluble in water. • The attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) are relatively strong • Halogenoalkanes are only slightly polar and do not effectively break the hydrogen bonds between water molecules. • Halogenoalkanes are soluble in non polar or less polar organic solvents such as alcohol, ether, and benzene . 37

Carbon is Unique • More than 90% of all known compounds are carbon compounds and therefore organic. • Living systems are all carbon based • Carbon has unique characteristics that make multiple compounds possible.

Carbon Characteristics • Electron configuration 1s2 2s2 2p2 • Four valence electrons • Hybridization • Forms four bonds with other atoms including other carbon atoms • Able to catenate – form chains and rings • Able to form multiple bonds

Bonding in ethene

Organic Chemistry