Ch. 25: Organic Chemistry

1. Ch. 25: Organic Chemistry • Organic chemistry is the branch of chemistry that studies carbon compounds. • Organic compounds were originally thought to be synthesized only in living organisms until… • - Friedrich Wöhler synthesized urea (1828) to dispel the idea. • Now many organic compounds are synthesized: synthetic oil, gasoline, rubber, plastics… • General Characteristics of Organic Compounds • Structure: Carbon can exhibit three different types of hybridization… • - sp3 hybridized carbons for tetrahedral geometries • - sp2hybridized carbons for trigonal planar geometries • - sp hybridized carbons for linear geometries

2. Carbon’s Hybridization Remember: Carbon’s hybridization relates to the # of atoms that it is bonded to… 4 atoms= sp3 3 atoms= sp2 2 atoms= sp

3. The Stabilities of Organic Molecules • Stability: Carbon also forms strong bonds with itself. • Therefore, C can form stable long chain or ring structures. cyclopropane (unstable) CnH(2n) CnH(2n+2)

4. The Stabilities of Organic Molecules • Remember: • - Bond strength increases from single to double to triple bond. • Bond length decreases in the same direction. • Strongest & shortest bond  C ≡ C • Carbon and hydrogen have very similar electronegativities, so the C-H bond is essentially non-polar. • Therefore, compounds containing C-C and C-H bonds are stable. • Adding certain groups of atoms called “functional groups” introduces reactivity into organic molecules. • Solubility of Organic Compounds • Solubility in water varies: • Long nonpolar molecules are insoluble in water. • Polar groups that are added will increase solubility.

5. Introduction to Organic Compounds • The simplest type of organic compounds are the “alkanes”. • Alkanes contain only single bonds. • These compounds are referred to as “saturated hydrocarbons”…C is allowed to bond to the maximum number of hydrogen atoms. • Alkanes have the general formula: CnH(2n+2)

6. Isomers • “Straight chain” hydrocarbons have each C atom joined in a continuous chain. • In a straight chain hydrocarbon no one C atom may be attached to more than two other C atoms. • Straight chain hydrocarbons are not linear! • Each C atom is tetrahedral, so the chains are actually bent. • Branched chain hydrocarbons are possible for four or more C atoms, which give rise to “isomers”…same formula, different properties • Structural isomers are compounds having the same molecular formula but differ in the bonding arrangement of the atoms. • - Example: There are 2 hydrocarbons with the formula C4H10.

7. Structural Isomers

8. Structural Isomers Pentane (C5H12) has 3 structural isomers. • The # of isomers greatly increases with the # of carbon atoms: • Octane, (C8H18), has 18 isomers and decane, (C10H22), has 75 isomers!

9. Stereoisomers • Stereoisomers have the same formula and the same bonding of atoms, but they differ in the spatial arrangement of the atoms. • - Example: “Geometric Isomers” • Since the double bond cannot rotate, they are truly different!

10. Stereoisomers • Optical isomers (or enantomers) are also stereoisomers. They are molecules that are “nonsuperimposable mirror images” of each other. • They rotate plane polarized light differently. • In order to have an optical isomer, you must have a “chiral center”…a carbon atom with 4 different attached groups.

11. Optical Isomer 2-bromopentane

12. Optical Isomer Enantiomers of alanine, an amino acid

13. Optical Isomers • Here’s another example of optical isomers… • Consider the compound carvone: • - The one on the left is responsible for the odor and taste of caraway and dill. • - The one on the right is isolated from spearmint.

14. Unsaturated Hydrocarbons • Alkenes: Contain a double bond…C = C. • The double bond increases its reactivity. (Often times, elements are added to the compound via the double bond being broken.) • The simplest alkenes are H2C=CH2 (ethene) and CH3CH=CH2 (propene)…(their trivial names are ethylene and propylene.) • Alkenes are named in the same way as alkanes with the suffix -ene replacing the -ane in alkanes. • The location of the double bond is indicated by a number. • (We will get more into naming compounds later!)

15. Alkenes Geometric isomers (“cis-trans”) only occur in compounds with a double bond.

16. Unsaturated Hydrocarbons • Alkynes: Contain a triple bond…C ≡ C. • The triple bond increases its reactivity even more vs.C = C. • The simplest alkyne is HC≡CH (ethyne, or acetylene.) • When it is burned in the presence of oxygen (oxyacetylene torch) the temperature is about 5300º F! • Alkynes are named in the same way as alkenes with the suffix -yne replacing the -ene for alkenes.

17. Unsaturated Hydrocarbons • Aromatic hydrocarbons: carbon atoms are connected in a planar ring structure. (They have a distinct odor to them.) • - The carbons are linked by both σ and -bonds. • - The best known example is benzene (C6H6).

18. Aromatic Hydrocarbons Benzene (C6H6). The delocalized -bonds makes aromatic compounds rather stable.

19. Summary of Hydrocarbons

20. Naming Organic Compounds • All organic molecule names have three parts: • - Prefix, which tells the nature of the substituents. • - Base, which gives the number of carbons. • Suffix, which gives the family (alkanes, alkenes, etc.) • Rules for naming compounds are given by the International Union for Pure and Applied Chemistry (IUPAC). • We will only do one example in these notes for practice, but we will have more to do together later on!

21. Naming Organic Compounds • Name this organic compound: • Step 1:Find the longest chain and use it as the base name of the compound. 7 C’s heptane

22. Naming Organic Compounds • Step 2: Number the carbon atoms starting with the end closest to the substituents. • - Groups attached to the main chain are called substituents. • - The preferred numbering will give substituents the lowest numbers. 4 5 6 7 1 2 3 2- ________ 4- _________heptane

23. Naming Organic Compounds • Step 3: Name and give the location of each substituent. • - When two or more substituents are present list them in alphabetical order! • - A substituent group formed by removing an H atom from an alkane is called an alkyl group. • Alkyl groups are named by replacing the –ane ending with -yl. • Examples: CH4 is methane, and a –CH3 group is a methyl group. • –CH2CH3 would be an ethyl group. • –CH2CH2CH3 would be a propyl group. • When there are two or more of the same substituent, the number of that type of substituent is indicated by a prefix… • Examples: “dimethyl” indicates two methyl group substituents. • “dichloro” indicates two chlorine substituents.

24. Naming Organic Compounds • Let’s name the substituents… methyl- 4 5 6 7 1 2 3 ethyl- Put them in alphabetical order!!! 4-ethyl-2-methylheptane (Again, we will do more practice problems later.)

25. Functional Groups • Hydrocarbons are relatively unreactive. For an organic molecule to be reactive it needs something additional… • • A site of “reactivity” in an organic molecule is called a functional group. Functional groups control how a molecule functions. • These groups determine the chemistry of the molecule. • Functional group containing molecules can either be saturated… (single bonds only-- alcohols, ethers, amines, etc.) or unsaturated… (double bonds involved--carboxylic acids, esters, amides, etc.) • (We usually use R to represent alkyl groups… methyl, ethyl, etc.)

26. Functional Groups

27. Functional Groups

28. Properties of Functional Groups • Alcohols (R-OH): do not liberate OH− ions in solution like bases • Examples: CH3OH, methanol, is used as a gasoline additive and a fuel. • CH3CH2OH, ethyl alcohol, or ethanol, is “booze”.

29. Properties of Functional Groups Ethers (R-O-R′): used as anesthetics, solvent for gums, fats, and wax. Example: diethyl ether (CH3CH2 – O – CH2CH3) Aldehydes:Example: methanal (or formaldehyde) (CH2=O) is used as a preservative for biological specimens…it coagulates proteins. Ketones:Examples: fingernail polish remover is propanone [or acetone… (CH3)2C=O ] used as solvents for plastics; manufacture of cordite, an explosive; carvone, (the optical isomer example) is a ketone.

30. Properties of Functional Groups Carboxylic Acids (COOH): found in fruits… (ascorbic acid, or vitamin C; malic acid is in apples); Acetic acid (CH3COOH) is vinegar; Formic acid (HCOOH) was first obtained by distillation of red ants (responsible for the sting in ant bites.) Carboxylic acids are very weak acids/weak electrolytes.

31. Properties of Functional Groups • Esters (RCOOR’): often have distinctive odors: ethyl butyrate (C3H7COOC2H5) has the odor of pineapple; butyl acetate (CH3COOC4H9) has the odor of bananas. • Other Examples: • benzocaine (in sun burn lotions); • ethyl acetate (nail polish remover); • vegetable oils; • polyester thread; • aspirin…which has an ester functional group as well as a carboxylic acid functional group (see previous slide)

32. Properties of Functional Groups • Amines and Amides (R3N): • Amines are organic bases where R may be H or a hydrocarbon group. • Just as alcohols, (R – OH), can be thought of organic forms of water, (H – OH) amines can be thought of organic forms of ammonia, NH3. • Example: ethylamine, (CH3CH2NH2) • Amines are among the most noxious-smelling of all organic compounds. • Amides are composites of carbonyl and amine functional groups: • Amides are the key functional group for amino acids which link up to make proteins. carbonyl group amine group

33. Functional Groups Practice Problem: What functional groups do you see highlighted in these sugars? Answers: * Glucose is an aldehyde. * Fructose is a ketone. * Both contain multiple hydroxyl (or alcohol) functional groups too.