1. Rayat Shikshan Sanstha's S.M. JOSHI COLLEGE, HADAPSAR, PUNE-28
A SEMINAR ON
HYDROCARBONS
Prof. MADHURI TUPE (Assistant Professor)
Department of Chemistry

2. Organic chemistry is the chemistry of carbon.

3. Hydrocarbons
Organic molecules contain carbon combined with other elements.
Organic molecules are grouped into families
Members of a family share common structural, physical, and chemical characteristics.
There are four families that contain molecules made of only carbon and hydrogen.
Hydrocarbons
Alkanes
Alkenes
Alkynes
Aromatics

4. Hydrocarbons

5. Alkanes
Alkanes are hydrocarbons that contain only carbon-carbon single bonds.
Every carbon atom participates in 4 single bonds, either to another carbon or to a hydrogen.
Every hydrogen atom is bonded to a carbon by a single bond.

6. Alkanes
Alkanes are hydrocarbons that contain only carbon-carbon single bonds.

7. Alkanes
Alkanes in which the carbons are connected in a straight chain are called normal alkanes.
Alkanes that are branched are called branched chain alkanes.
n-hexane
2-methyl-pentane

8. Alkanes
For a discusion on the structure of alkanes, see the Unit 2 Elaboration - Alkane Structure

9. Alkanes Alkanes, along with the other hydrocarbons, are non-polar.
They interact with each other only through London dispersion forces.
This is why they have relatively low boiling and melting points.

10. Alkanes
They interact with each other only through London dispersion forces.
Note how the boiling points increase with molecular weight.

11. Alkanes Alkanes, cannot be named based on their molecular formulas
For example, all of the molecules shown below share the same molecular formula, C6H14 (hexacarbon tetradecahydride?)
n-hexane
2-methyl-pentane
3-methyl-pentane
2,2-dimethylbutane
2,3-dimethylbutane

12. Alkanes
Organic chemists use a systematic set of rules, called the IUPAC rules, to name organic molecules based on their structural formulas instead of their chemical formulas.
n-hexane
2-methyl-pentane
3-methyl-pentane
2,2-dimethylbutane
2,3-dimethylbutane

13. Constitutional Isomers
When two or more molecules share the same molecular formula, but have different atomic connections, they are called constitutional isomers.
n-hexane
2-methyl-pentane
3-methyl-pentane
2,2-dimethylbutane
2,3-dimethylbutane

14. Conformations Carbon-carbon single bonds are free to rotate
This leads to different shapes for some molecules
These should not be confused with isomers.

15. Conformations
All of the 3-dimensional models shown below are for the n- butane.
They were generated by rotating the central carbon-carbon bond.
They all share the same structural formula.

16. Conformations
All of the 3-dimensional models shown below are for the n- butane.
They were generated by rotating the central carbon-carbon bond.

17. Conformations
Switching from one conformation to another does not require the breaking and making of covalent bonds.
Switching from one isomer to another does require the breaking and making of covalent bonds.
n-butane
2-methylpropane

18. Cycloalkanes
When there are three or more carbons in a straight chain, the ends can be joined to make rings.
In naming these molecules, the prefix cyclo- is used to indicate the ring:
Skeletal structural formulas are used to represent the rings in structural formulas:

19. Cycloalkanes
In naming these molecules, the prefix cyclo- is used to indicate the ring:
As Parent Chain
As Substituent Group

20. Cycloalkanes
The carbon-carbon single bonds for the carbons in a ring are no longer free to rotate.
This leads to a new type of isomer
Since the two structures share the same name, they are not constitutional isomers.

21. Cycloalkanes
Isomers which share the same atomic connections, and therefore, the same IUPAC name are called stereoisomers.
When this occurs due to restricted rotation about a covalent bond, they are called geometric isomers
The prefix cis- and trans- are used to distinguish geometric isomers.

22. Alkenes, Alkynes & Aromatic Compounds
The remaining three families of hydrocarbons are unsaturated.
Alkanes are saturated, which means they contain the maximum number of hydrogens per carbon.
For alkanes CnH(2n+2)
Alkenes, Alkynes and Aromatics are unsaturated, which means they contain less than the maximum number of hydrogens per carbon.
Structurally, this means that they have carbon-carbon double or triple bonds

23. Alkenes, Alkynes & Aromatic Compounds
Alkenes are hydrocarbons that contain at least 1 carbon- carbon double bond.
Examples:

24. Alkenes, Alkynes & Aromatic Compounds
Alkynes are hydrocarbons that contain at least 1 carbon- carbon triple bond.
Examples:

25. Alkenes, Alkynes & Aromatic Compounds
Aromatics are unsaturated ring molecules
They are often drawn to look like alkenes, but they behave much differently than alkenes.
They have an alternating pattern of double and single bonds within a ring.
Benzene is an example

26. Alkenes, Alkynes & Aromatic Compounds
The physical properties of all hydrocarbons are the same
The have essentially one noncovalent interaction, which isthe London dispersion force.
They have no electronegative atoms and therefore have
No ion/ion interactions
No dipole/dipole interactions
No hydrogenbonding interactions

27. Alkenes, Alkynes & Aromatic Compounds
Naming of Alkenes and Alkynes work the same as for alkanes, with these added rules:
The parent chain must include both carbons in all double and triple bonds.
Pick the longest chain that also contains all double and triple bonds
The -ene ending is used of alkenes
The -yne ending is used for alkynes.
The number of the first carbon in the double or triple bond is included in the name to locate the double or triple bond.
Number the parent chain from the end that is closes to the first double or triple bond.

28. Alkenes, Alkynes & Aromatic Compounds
Naming of Aromatics is based on benzene:
When the molecule is build on benzene, the parent name is “benzene”.
There are also many common names used to describe aromatic compounds.

29. Alkenes, Alkynes & Aromatic Compounds
Naming of Aromatics is based on benzene:
Aromatic compounds can contain multiple aromatic rings

30. Alkenes, Alkynes & Aromatic Compounds
There are many aromatic molecules found in biology
Some aromatic compounds contain nitrogen and oxygen atoms
For example, the nucleotide base Adenine, which is used to make DNA and RNA

31. Alkenes, Alkynes & Aromatic Compounds
Like cycloalkanes, some alkenes can have cis and trans isomers
This is due to restricted rotation about the double-bond.
Not all double bonds produce cis and trans isomers
Each carbon participating in the double bond must have two different substituents attached to them
A ≠ B AND X ≠ Y

32. Alkenes, Alkynes & Aromatic Compounds
Like cycloalkanes, some alkenes can have cis and trans isomers

33. Alcohols, Carboxylic Acids & Esters
In addition to the four families of hydrocarbons, there are also many other families of organic molecules.
These other families include elements other than carbon and hydrogen.
They exhibit a wide range of chemical and physical properties.
The families are distinguished by a group of atoms called a functional group

34. Alcohols, Carboxylic Acids & Esters
Functional Group
“A functional group is an atom, group of atoms or bond that gives a molecule a particular set of chemical and physical properties”

35. Alcohols, Carboxylic Acids & Esters
The carbon-carbon double bonds found in alkenes is an example of a functional group.
A chemical property of a double is that it will absorb hydrogen in the hydrogenation reaction.

36. Alcohols, Carboxylic Acids & Esters
We look now at three families that are distinguished by a functional group that contains the element oxygen.
Alcohols
Members of the alcohol family contain a hydroxyl group.
The hydroxyl group comprises an oxygen with one single bond to a hydrogen and another single bond to an alkane- type carbon
hydroxyl group
An alkane-type carbon atom
ethanol

37. Alcohols, Carboxylic Acids & Esters
We look now at three families that are distinguished by a functional group that contains the element oxygen.
Carboxylic acids
Members of the carboxylic acid family contain a carboxylic acid group
The carboxylic acid group comprises a hydroxyl group connected to a carbonyl group: +
carbonyl group
hydroxyl group
carboxylic acid group

38. Alcohols, Carboxylic Acids & Esters
The present of the hydroxyl group next to the cabonyl group completely changes it properties.
The alcohol hydroxyl group and the carboxylic acid hydroxyl group are chemically quite different, which is why molecules that have the carboxylic acid group are placed in a separate family from the alcohols.
Later in the semester we will learn about some of these chemical differences. +
carbonyl group
hydroxyl group
carboxylic acid group

39. Alcohols, Carboxylic Acids & Esters
The carboxylic acid group can be attached to a hydrogen, an alkane-type carbon, or an aromatic-type carbon:
methanoic acid
(formic acid)
propanoic acid
benzoic acid

40. Alcohols, Carboxylic Acids & Esters
We look now at three families that are distinguished by a functional group that contains the element oxygen.
Esters
Chemically, esters can be synthesized by reacting a carboxylic acid with and alcohol:
carboxylic acid
alcohol
ester
water

41. Alcohols, Carboxylic Acids & Esters
We look now at three families that are distinguished by a functional group that contains the element oxygen.
Esters
Chemically, esters can be synthesize by reacting a carboxylic acid with and alcohol:
Ethyl propanoate

42. Alcohols, Carboxylic Acids & Esters
The carboxylic acid group can be attached to a hydrogen, an alkane-type carbon, or an aromatic-type carbon:
methanoic acid
(formic acid)
propanoic acid
benzoic acid

43. Alcohols, Carboxylic Acids & Esters
As we saw with the hydrocarbons, the physical properties of organic molecules depend on the noncovalent intermolecular interactions which attract one one molecule to another.
With hydrocarbons, there is only one type of noncovalent interaction:
Induced dipole/Induced dipole (London dispersion force)
The presence of the electronegative oxygen makes alcohols, carboxylic acids and esters polar molecules, these families, therefore, have at least two types of noncovalent interactions:
Dipole/Dipole

44. Alcohols, Carboxylic Acids & Esters
As we saw with the hydrocarbons, the physical properties of organic molecules depend on the noncovalent intermolecular interactions which attract one one molecule to another.
Alcohols and Carboxylic acids also have a hydroxyl group with a hydrogen bonded to an oxygen. This allows them to form hydrogen bonds with each other. Therefore, carboxylic acids have at least three different noncovalent interactions:
Induced dipole/Induced dipole (London dispersion force)
Dipole/Dipole
Hydrogen bond

45. Alcohols, Carboxylic Acids & Esters
To summarize, the types of noncovalent interact ions that each family can participate in include:
Hydrocarbons (Alkanes, Alkenes, Alkynes & Aromatics)
Induced dipole/Induced dipole (London dispersion force)
Esters
Dipole/Dipole
Alcohols & Carboxylic acids
Hydrogen bond

46. Alcohols, Carboxylic Acids & Esters
These interactions are illustrated in Figure 4.23 of your textbook.
alcohols
carboxylic acids
esters

47. Alcohols, Carboxylic Acids & Esters
Boiling points are a good measure of the strength of the noncovalent interactions between molecules.
The stronger the interactions, the higher the boiling point will be.
Since all molecules have the London dispersion interaction, the boiling points of molecules is expected to increase with temperature.
The next slide shows a chart using the data found in Table 4.7 of Raymond, in which the boiling points for alcohols, carboxylic acids and esters are plotted against molecular weight.

48. Alcohols, Carboxylic Acids & Esters
As expected, the boiling points for members of all three families increases with molecular weight due to the London dispersion interactions.
For a given molecular weight, the alcohols and carboxylic acids have a higher boiling point than esters, this is because they can form hydrogen bonds and esters cannot.
The carboxylic acids have a slightly higher boiling point than alcohols, because they can form two hydrogen bonds with a neighboring molecule (See Figure 4.23 in Raymond)
Molecular Weight {g/mol}
Boiling Point {°C}

49. Alcohols, Carboxylic Acids & Esters
Another distinguishing characteristic of many of the families is odor.
You nose is actually a highly sensitive chemical detector.
The members of different families can interact differently with the receptors in your nose to produce smells that are characteristic of the families they belong to.
For example:
Carboxylic acids produce the pungent, sometime unpleasant odors associated with ripe cheeses, rancid butter and vomit.
Esters, on the other hand, produce the sweet, often pleasant order associated with flowers, perfumes and various natural and artificial flavorings. The next slide shows Figure 4.24 from Raymond, which gives some specific examples.

50. Alcohols, Carboxylic Acids & Esters
Examples of some flavorable esters:

51. THANK YOU