organic chemistry

CHAPTER 10 & 15Organic Chemistry

K. DunlapChem 104 Spring

the study of the compounds of carbon.

•Organic compounds are made up of carbon and only a few other elements.•chief among these are hydrogen, oxygen, and nitrogen•also present are sulfur, phosphorus, and halogens (fluorine, chlorine, bromine, or iodine)

Organic Chemistry

Why is organic chemistry a separate discipline within chemistry?

1) Historical:1) Historical: scientists at one time believed that a “vital force” present in living organisms was necessary to produce an organic compound.

•The experiment of Wöhler in 1828 was the first in a series of experiments that led to the demise of the vital force theory.

2) The sheer number of organic compounds2) The sheer number of organic compounds – Chemists have discovered or made over 10 million

organic compounds and an estimated 100,000 new ones are discovered or made each year.

– By comparison, chemists have discovered or made an estimated 1.7 million inorganic compounds.

– Thus, approximately 85% of all known compounds are organic.

3) The link to biochemistry3) The link to biochemistry– Carbohydrates, lipids, proteins, enzymes, nucleic

acids, hormones, vitamins, and almost all other chemicals in living systems are organic compounds.

Carbon-Carbon BondsCarbon-Carbon Bonds

Models of ALKANESModels of ALKANES

Predict the shape of methane, CH4

– The Lewis structure shows carbon surrounded by four regions of electron density.

– the four regions radiate from carbon at angles of 109.5°, and the shape of the molecule is tetrahedral.

– The measured H-C-H bond angles are 109.5°.

MOLECULAR MOLECULAR GEOMETRYGEOMETRY

MOLECULAR MOLECULAR GEOMETRYGEOMETRY

Valence-Shell Electron-Pair Repulsion (VSEPR) Model

– valence electrons of an atom may be involved in forming bonds or may be unshared.

– each combination creates a negatively charged region of electrons around the nucleus.

– because like charges repel each other, the various regions of electron density around an atom spread so that each is as far away from the others as possible.

Electron Pair GeometriesElectron Pair GeometriesSee Active Figure 8.5See Active Figure 8.5

Predict the shape of methane, CH4

– The Lewis structure shows carbon surrounded by four regions of electron density.

– According to the VSEPR model, the four regions radiate from carbon at angles of 109.5°, and the shape of the molecule is tetrahedral.

– The measured H-C-H bond angles are 109.5°.

Predict the shape of ammonia, NH3

– nitrogen is surrounded by four regions of electron density (3 with single pairs of electrons, and 1 with an unshared pair of electrons).

– According to the VSEPR model, the four regions radiate from nitrogen at angles of 109.5°, and the shape of the molecule is pyramidal.

– The measured H-N-H bond angles are 107.3°

Predict the shape of water, H2O

– The Lewis structure shows oxygen with four regions of electron density (2 regions with single pairs of e-, and 2 with unshared pairs of e-.

– According to the VSEPR model, the four regions radiate from oxygen at angles of 109.5°, and the shape of the molecule is bent.

– The measured H-O-H bond angle is 104.5°.

Predict the shape of formaldehyde, CH2O

– The Lewis structure shows carbon surrounded by 3 regions of electron density; 2 with single pairs of e- and one with 2 pairs of e- forming the double bond to oxygen.

– According to the VSEPR model, the three regions radiate from carbon at angles of 120°, and the shape of the molecule is planar (trigonal planar).

– The measured H-C-H bond angle is 116.5°.

Predict the shape of ethylene, C2H4

– The Lewis structure shows carbon surrounded by 3 regions of e- density; 2 with single pairs of e- and 1 with two pairs of electrons forming the double bond to the other carbon.

– According to the VSEPR model, the three regions radiate from carbon at angles of 120°, and the shape of the molecule is planar (trigonal planar).

– The measured H-C-H bond angle is 117.2°.

Predict the shape of acetylene, C2H2

– The Lewis structure shows carbon surrounded by 2 regions of electron density; one region with a single pair of e-, and the other one with three pairs of e- forming the triple bond to carbon.

– According to the VSEPR model, the two regions radiate from carbon at an angle of 180°, and the shape of the molecule is linear.

– The measured H-C-C bond angle is 180°.

Bond PropertiesBond Properties• What is the effect of bonding and

structure on molecular properties?

Free rotation Free rotation around C–C single around C–C single bondbond

No rotation around No rotation around C=C double bondC=C double bond

Bond OrderBond Order

Double bond Double bond = 2= 2ndnd order orderDouble bond Double bond = 2= 2ndnd order order Single bond = 1Single bond = 1stst order orderSingle bond = 1Single bond = 1stst order order

Triple Triple bond 3bond 3rdrd orderorder

Triple Triple bond 3bond 3rdrd orderorder

AcrylonitrileAcrylonitrileAcrylonitrileAcrylonitrile

Bond LengthBond LengthBond LengthBond Length

• Bond length is the distance between the nuclei of two bonded atoms.

Bond length depends on bond order.

Bond distances measured in Bond distances measured in Angstrom units where 1 A = Angstrom units where 1 A = 1010-2 -2 pm.pm.

Bond distances measured in Bond distances measured in Angstrom units where 1 A = Angstrom units where 1 A = 1010-2 -2 pm.pm.

Bond Length

• —measured by the energy required to break a bond. See Table 8.9.

BOND Bond dissociation enthalpy (kJ/mol)

H—H 436

C—C 346

C=C 602

CC 835

NN 945

The GREATER the number of bonds (bond order) the The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.HIGHER the bond strength and the SHORTER the bond.The GREATER the number of bonds (bond order) the The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.HIGHER the bond strength and the SHORTER the bond.

Bond StrengthBond Strength

• an atom or group of atoms within a molecule that shows a characteristic set of predictable physical and chemical properties.

• Functional groups are important because:

1) They undergo the same types of chemical reactions no matter in which molecule they are found.

2) To a large measure they determine the chemical and physical properties of a molecule.

3) They are the units by which we divide organic compounds into families.

4) They provide the basis on which we derive names for organic compounds.

What are Functional Groups?

AlcoholsAlcohols

contains an OH (hydroxyl) group bonded to a

tetrahedral carbon atom. For example, ethanol:

Alcohols may be primary (1°), secondary (2°), or tertiary (3°)

Organic AlcoholsOrganic Alcohols

Amines:

• a compound containing an amino group.amino group.– the amino group may be primary (1°), secondary

(2°), or tertiary (3°).

Amines Amines

• Both contain a C=O (carbonyl) group.C=O (carbonyl) group.– Aldehyde:Aldehyde: contains a carbonyl group bonded to a

hydrogen; in formaldehyde, the simplest aldehyde, the carbonyl group is bonded to two hydrogens.

Aldehydes and Ketones:

Ketone:Ketone: contains a carbonyl group bonded to two carbon atoms.

Aldehydes and KetonesAldehydes and Ketones

• a compound containinga compound containing a -COOH (carboxyl: carbcarbonyl + hydroxyloxyl) group. – In a condensed structural formula, a

carboxyl group may also be written -CO2H.

Carboxylic Acids:

Carboxylic Acids Carboxylic Acids

• a derivative of a carboxylic acid in which the H of the carboxyl group is replaced by a carbon group.

Carboxylic esterCarboxylic ester:

Esters Esters

Chiral Molecules

• Is a molecule that has 4 different atoms or groups attached to the carbon

•have the same molecular formula, and same order, but differ only in the spatial arrangement

• Mirror images

No mater how they are rotated they can not be superimposed

In Nature, chiral molecules exist almost exclusively in one form of the other

10.6

Chiral molecules have 4 different groups attached to a central atom.

A chiral molecule and its non-superimposable mirror image are a special kind of isomer called enantiomers.

Enantiomers have identical physical properties.

The only way we can tell them apart is by seeing their effect on plane polarized light.

10.6

However, the body can tell them apart. These two enantiomers may have very different actions in the human body.

One enantiomer fits into a receptor site, while the other does not. The molecule on the right will have (possibly) no affect on the human body.

The R,S System

• Because enantiomers are different compounds, each must have a different name.– Here are the enantiomers of the over-the-counter drug

ibuprofen.

– The R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other.

The R,S System• The first step is to establish priority.

– Priority is based on atomic number.– The higher the atomic number, the higher the priority.

The R,S System• To assign an R or S configuration:

1. Assign a priority from 1 (highest) to 4 (lowest) to each group bonded to the stereocenter.

2. Orient the molecule in space so that the group of lowest priority (4) is directed away from you; the three groups of higher priority (1-3) then project toward you.

3. Read the three groups projecting toward you in order from highest (1) to lowest (3) priority.

4. If reading the groups 1-2-3 is clockwise, the configuration is RR; if reading them is counterclockwise, the configuration is SS.

Optical Activity– Dextrorotatory:Dextrorotatory: clockwise rotation of the

plane of plane-polarized light.– Levorotatory:Levorotatory: counterclockwise rotation of

the plane of plane-polarized light.

Specific rotation:Specific rotation: the observed rotation of an optically active substance at a concentration of 1 g/mL in a sample tube 10 cm long.

10.6

Consider the two enantiomers above. Dextromethorphan is a safe cough suppressant. Levomethorphan is an addictive opiate.