Organic molecules

Nomenclature and properties of organic substances

Learning objectives:

• Explain the differences between organic and inorganic compounds.

• Explain the special characteristics of carbon that make organic molecules unique.

• Explain and give examples of structural isomers.• Describe the various functional groups and use these

functional groups to name and draw structural formulas for various organic molecules.

• Relate the carbon chain length and various functional groups present to the properties of the organic molecules.

What are organic molecules and how do they differ from inorganic

molecules?• Organic molecules are molecules that contain

carbon. (With carbonates, CO, and CO2 being exceptions.)

• Inorganic molecules are all other compounds.

• NOTE: Organic substances are called molecules, whereas, inorganic are usually compounds (although they can be molecules). What does this imply about the bonding that takes place between atoms of organic substances?

Let’s compare organic and inorganic substances!

• Organic– Usually contain covalent

bonding within the molecule

– Generally weak forces between molecules

– Found usually as gases, liquids, or low M.P. solids

– Usually flammable– Low solubility in water– Nonconductors– Slow rate of chemical

reactions

• Inorganic– Usually contain ionic

bonding within the compound

– Generally strong forces between formula units

– Found usually has high M.P. solids

– Usually nonflammable– High solubility in water– Conductors– Usually fast rate of

chemical reactions

Test Yourself

1) Which of the following are organic molecules?

– NaCl - C8H18

– Water (H2O) - CaCO3

– C6H6 (Benzene) - CO

– A flammable substance– A substance with a low boiling point– A substance with strong intermolecular forces– A substance that is water soluble

Why the difference?

• To explain the difference, we first must review some basic theories from first level chemistry.

• Recall: – Lewis (e- dot) Theory – A theory that explains nicely why atoms

bond in particular whole number ratios (Ex: NaCl, H2O).

– VSEPR Theory – A theory that explains nicely the observed molecular shapes (Ex: Bent structure of a water molecule as opposed to the predicted linear structure.)

– Molecular Orbital Theory – Explains the bonding between atoms to give the unexpected shapes of certain molecules (Ex: the tetrahedral arrangement of carbon atoms in a diamond.)

The difference is the carbon-carbon bonding!!

• Carbon is unique in that it has 4 valence electrons, 2s and 2p valence electrons (therefore, you may think it will likely bond with two other atoms through single bonds to pair up the two unpaired electrons.) – However, we observe this NOT to be the case for organic molecules. We observe 4 single bonds and the most common tetrahedral geometry.

• Why???

M.O. theory explains uniqueness of carbon

• Recall: The 2s and 2p orbitals hybridize and form four sp3 hybrid orbitals, intermediate in energy to the 2s and 2p orbitals. This results in 4 unpaired electrons and is responsible for the observed tetrahedral geometry.

• Carbon also likes to bond with itself (Recall the % ionic character of zero due to no electronegativity difference between bonded atoms, and therefore the purest non-polar covalent bond possible!). Some of these carbon-carbon linkages can be enormously long (these are macromolecules!)– The covalent bonding is the primary reason for the weak forces between

molecules, and therefore for the lower melting and boiling points (easier to break the molecules apart). This also explains the low solubility in water (Recall: “Like dissolves Like” and water is a VERY polar molecule.

Structural isomers?

• Due to the unique carbon-carbon bonding and the various possibilities, structural isomers (substances with the same molecular formulas, but different bonding arrangement of atoms) are more common for organic compounds than inorganic compounds.

Example of structural isomers

• Ethanol (Ethyl alcohol)

– Formula: C2H6O

– Liquid at room temperature

• Dimethyl ether

– Formula: C2H6O

• Gas at room temperature

Test Yourself!!!

• See the Learning Check 11.3 on Page 347 of the textbook.

How are all the organic substances organized?

• They are characterized according to characteristic structural features called functional groups. Some examples are as follows: – A carbon – carbon double bond (called alkenes)– A carbon – carbon triple bond (called alkynes)– Carbonyls ( a carbon atom double bonded to an

oxygen atom, of which there are various sub classes)

– For a complete listing, see the handout provided or Table 11.2 on page 348 of the textbook.

What is Organic nomenclature?

• Nomenclature involves writing formulas for and drawing structures for a substance. For organic nomenclature, this involves carbon molecules.

• When drawing structures, there are two types:– 1) Expanded structural formulas – show ALL

covalent bonds– 2) Condensed structural formulas – Show

only specific bonds

What are hydrocarbons?

• The most common non-carbon atom bonded to a carbon atom in an organic molecule is the hydrogen atom. For this reason, most organic molecules are also called hydrocarbons.

Saturated vs. Unsaturated hydrocarbons?

• Saturated hydrocarbons are hydrocarbons with the maximum number of atoms bonded to the carbons (These atoms are usually hydrogen atoms or halogens). These are called alkanes (contain NO carbon – carbon double or triple bonds)

• Saturated hydrocarbons usually have a molecular formula of CnH2n+2 (with n being the number of carbon atoms in the molecule.)

Alkane nomenclature

• Use the following prefixes when naming alkanes (they vary depending on the carbon chain length):– 1 carbon: meth - 6 carbons: hex– 2 carbons: eth - 7 carbons: hept– 3 carbons: prop - 8 carbons: oct– 4 carbons: but - 9 carbons: non– 5 carbons: pent - 10 carbons: dec

– For alkanes, use the above prefix and the “ane” ending.

– For Ex: CH4 – methane or C3H8 – propane

– For a complete listing see table 11.4 on page 356.

Organic Nomenclature of branched alkanes

• Branched alkanes are alkanes that do NOT form a long continuous chain of carbon atoms. These consist of alkyl groups. (These are alkanes missing a hydrogen atom and serve as substituents of a longer parent chain.)

• Unbranched alkanes usually contain the n- prefix to indicate an unbranched chain. The iso- prefix is used to indicate a branched alkane with all carbon atoms on the long continuous chain except one and that one is branched next to end of the parent chain. – Ex: n-butane iso-butane

CH3

I

– CH3CH2CH2CH3 CH3CHCH3

How are alkyl groups named?

• Alkyl groups are named by identifying the alkane root of the alkyl group, dropping the “ane” ending and adding “yl”.

• For example:• CH4 is methane -CH3 is methyl• CH3CH3 is ethane -CH2CH3 is ethyl

• For a listing of some common alkyl groups, see Table 11.5 on page 357.

Group  CH3–  C2H5–

 CH3CH2CH2–   (CH3)2CH–   CH3CH2CH2CH2–   (CH3)2CHCH2–   CH3CH2CH(CH3)–   (CH3)3C–    

Name 

Methyl 

Ethyl 

Propyl Isopropyl

 Butyl  Isobutyl  sec-Butyl  t-butyl

Mini Quiz• Name the following alkyl groups:

CH3CH2CH2-

CH3CH2CH2CH2CH2-

CH3

I

CH3CHCH2-

CH3

I

CH3CCH3

I

Naming alkanes – using IUPAC rules

• Step 1) Name the longest chain.• Step 2) Number the longest chain. If there are attached

carbon groups, number so that they give the lowest number to any attached carbon.

• Step 3) Locate and name any attached alkyl groups.• Step 4) Combine the longest chain and the braches into

one name. Use numbers & dashes to indicate the position of the branches on the longest chain (called the parent chain).

• Step 5) If there are more than one of the same branched group, use commas between the numbered locations and use the prefixes di, tri, tetra, penta, etc… to indicate the correct number of branched groups.

Examples:• Name the following alkanes:

1) CH3CH2CH2CH2CHCH3

I CH3

CH2CH3

I 2) CH3CH2CH2CHCH2CHCH3

I CH2CH3

CH3

I 3) CH3CH2CH2CHCHCHCH3

I I CH CH3

/ \ CH3 CH3

Now complete the self quiz by linking to the site below –

complete quiz 1 at this time.

Click here for the self-quiz

http://www.chembio.uoguelph.ca/educmat/chm19104/organic_nomenclature_quizzes.htm

Drawing structural formulas

• To draw the structural formula for an organic substance, simply reverse the previous process.

• Examples: Draw structures for:

1) 2,2,4-trimethylpentane

2) 3-isopropylhexane

3) 3-ethyl-2,4-dimethylheptane

Properties of alkanes

• Alkanes are NOT water soluble (due to the non-polar covalent bonding).

• As the length of the chain increases, the boiling point increases (less likely to be a gas at room temperature) and consequently the density increases.

Test Yourself

• Which has a higher B.P., propane or pentane?

• Is butane a liquid or gas at room temperature?

• Is octane more or less dense that pentane?

Cycloalkanes

• Cycloalkanes are alkanes containing rings of carbon atoms.

• These are named using the same set of rules already mentioned, only adding the prefix cyclo- before the alkane name.– If there is only one branched group, no number is needed to

indicate its location on the parent chain.– If there are more than one branched groups, then number

beginning with the first group alphabetically and proceed around the ring in a direction that gives the lowest numbering possible.

– See text or class notes for examples.

Special isomers

• In striaght chain alkanes, there is free rotation of the carbon atoms. This is NOT possible in cycloalkanes. – Therefore, cycloalkanes have tighter C-C bonds and

are harder to break, making them less reactive!– Due to restrictive nature of C-C bonds in

cycloalkanes, they can exist as geometric isomers (Molecules with restricted C-C bonds that have different 3-D arrangements of atoms at the restricted C-C site.) These types of isomers are also common for alkenes (C=C bonding).

Two types of geometric isomers

• Cis isomers– Substituted groups

project in the same direction from the rigid C-C plane.

– Ex: cis-1,2-dimethylcyclopentane

• Trans isomers– Substituted groups

project in the opposite direction from the rigid C-C plane.

– Ex: trans-1,2-dimethylcyclopentane

Test Yourself

• Draw the following:– 1) 1,1-dimethylcyclobutane– 2) trans-1,2-dimethylcycobutane– 3) cis-1,3-dimethylcyclobutane– 4) cis-1,2-dichlorocyclohexane– 5) trans-1,3-dibromocyclopentane

Nomenclature of unsaturated hydrocarbons (alkenes and alkynes and aromatics.)

• Alkenes – contain one or more C-C double bonds.– When naming, change the ending of the name from “ane” to “ene” and

use the previously mentioned numbering system to indicate the location of the double bond. The longest chain MUST include the double bond. Have a general formula of CnH2n (for one double bond).

• Must consider cis & trans geometric isomers.

• Alkynes – contain one or more C-C triple bonds.– When naming, change the ending of the name from “ane” to “yne” and

use the previously mentioned numbering system to indicate the location of the triple bond. The longest chain MUST include the triple bond. Have a general formula of CnH2n-2 (for one triple bond).

• Aromatics – includes cycloalkenes and the simplest aromatic, benzene.

Try some alkene nomenclature

• Name the following:– CH3-CH=CH-CH3

CH3-CH2

I C=CH2

ICH3-CH2-CH2

CH3-CH-CH=CH2

I CH3

For more examples, see the handouts in class or try some additional exercises from the book.

Properties of alkenes

• As the length of the alkene chain increases, again we observe in increase in the boiling point and the density of the substance.

Aromatics

• Organic molecules that have a benzene derivative.

• Contains staggered double bonds with delocalized electrons.

Aromatic root names – memorize these!!

• toluene

More aromatics

• analine • phenol

Stereoisomers of aromatics

• Uses the following isomer nomenclature when there are only 2 derivatives on a benzene ring:

• 1 & 2 substituted are ortho (o-)

• 1 & 3 substituted are meta (m-)

• 1 & 4 substituted are para (p-)

Now complete the self quiz by linking to the site below –

complete quiz 2 at this time.

Click here & take quiz 2

http://www.chembio.uoguelph.ca/educmat/chm19104/organic_nomenclature_quizzes.htm

Prepare for a TEST!!!!

End of Chapter 11 & 12 review questions are a good review for

the test!!