Organic Chemistry Help (page 3)
Introduction to Organic Chemistry
Organic chemistry, based originally on the study of living things like molds, plants, algae, red blood cells, gnats, and elephants, to name a few, is focused on compounds that include carbon. It is estimated that greater than 95% of all known chemicals contain carbon. At last count, there were over two million known organic compounds, nearly twenty times more than all the other known chemicals combined.
At one time, it was thought that organic compounds contained some type of “vital force” since they were once living organisms. However, when the organic compound urea (CH 4 ON 2 ) was made in the laboratory in 1828 by Friedrich Wohler out of ammonium carbonate it was the first time an organic compound had been made from inorganic materials off the shelf. Chemists, then, started thinking about the possibilities of organic molecules and their reactions in a whole different way.
This largest group of covalently bonded compounds makes up the central study of petroleum-based chemicals, plastics, synthetic fibers, and biological chemistry. Petroleum, also known as crude oil, is made up of organic compounds from the decomposed remains of plants and animals that died millions of years ago.
Organic chemistry is the chemistry of carbon.
The six electrons of carbon fill up the 1s, 2s, 2p orbitals. The four valence electrons tend to stay unpaired, allowing carbon to form four bonds. Carbon can form open chains, closed chains (rings), and a combination of open and closed chains.
The wide variety of carbon-containing compounds found in nature can be related to carbon’s talent. Carbon is able to form long-chain molecules like decane (C 10 H 22 ), branching macromolecules like natural rubber, and ring structures like menthol (from peppermint).
In the same group (IVA) as carbon, silicon is very much like carbon in atomic structure. It forms silicon-silicon covalent bonds, but since silicon is over double the size of carbon, the silicon bond lengths are longer and weaker. It is like a bridge between two river banks. The bridge across a 4 meter (12 foot) wide stream will be much stronger and more stable than one across an 8 meter (28 foot) stream, when the middle is not supported.
Figure 10.1 shows some common organic molecules.
The broad group of organic compounds called hydrocarbons are made up of, you guessed it, molecules containing only carbon and hydrogen. Hydrocarbons are perhaps the easiest molecules in all of chemistry to learn. Once you get the basics, the rest is a matter of plugging in additional element groups.
Hydrocarbons are divided into subgroups depending on how carbon and hydrogen have bonded. Those hydrocarbons made up of only single bonds between carbon and hydrogen are known as alkanes . When carbon forms a double bond with hydrogen in a molecule, the subgroup is known as alkenes . Similarly, molecules with triple bonds between carbon and hydrogen are known as alkynes . The two simplest members of the alkane group are methane and ethane.
Complete carbon bonding is seen in methane (CH 4 ). Carbon forms a tetrahedral (four-sided) compound with hydrogen, sharing electrons at bond angles of about 109 degrees. Figure 10.2 shows methane with bond angles shown.
Hydrocarbons react with other elements in specific ways. The bond to the hydrogen is broken and the reacting element is slipped into its place. For example, methane’s reaction with members of the halogen group (fluorine, chlorine, or bromine) produce halomethanes and a hydrogen halide.
The substitution reactions that occur between a hydrocarbon (methane, CH 4 ) and a halogen (chlorine, Cl) are shown.
CH 4 + Cl 2 ⇒ CH 3 Cl + HCl
The same reaction takes place with additions of the halogen to the singly substituted CH 3 Cl.
CH 3 Cl + Cl 2 ⇒ CH 2 Cl 2 + HCl
CH 2 Cl 2 + Cl 2 ⇒ CHCl 3 + HCl
CHCl 3 + Cl 2 ⇒ CCl 4 + HCl
Substitution reactions are simple and very common among organic compounds, but they can occur between other compounds too.
A substitution reaction between fluoromethane (CH 3 F) and potassium bromide (KBr) trades elements in the following reaction:
CH 3 F + KBr ⇒ CH 3 Br + KF
Naming Of Alkanes
The International Union of Pure and Applied Chemistry (IUPAC) method of naming alkanes is fairly simple. The names of all alkanes end in ane . The prefixes, based on the number of carbons in the molecule, make naming a lot like counting. Meth- (1 carbon), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hep- (7), oct- (8), non- (9), and dec- (10) are the basic prefixes. If you know these, you will be able to breeze through naming.
What are the names of the following alkanes?
(1) CH 4 , (2) C 3 H 8 , (3) C 6 H 14 , and (4) C 9 H 20 .
Did you get (1) methane, (2) propane, (3) hexane, and (4) nonane?
The names of functional groups resulting from alkanes after the loss of a hydrogen are called alkyl groups. Methane, then, becomes methyl , ethane becomes ethyl , and so on.
The naming of alkenes or double-bonded molecules is very much like that of the alkanes.
Alkenes are numbered using the following IUPAC rules:
(a) The base molecule name comes from the longest chain that contains the double bond. (So if the longest chain has 4 carbons, the base name would be butene.)
(b) The chain is numbered to include both carbons of the double bond. (Then CH 2 =CHCH 2 CH 3 is 1-butene.)
(c) The locations of the attached groups are numbered as to the carbon to which they are attached. (So 2-methyl-2-butene has a methyl group (−CH 3 ) on the second carbon of the double bonded-butene.)
(d) When naming 6-carbon ring molecules, the carbons are numbered clockwise around the ring. (So 2-methylbenzene has a methyl (CH 3 ) attached at the second carbon in the ring.)
Ethene (C 2 H 4 ) is an example of a simple double-bonded carbon molecule, with two hydrogen atoms bonding to each carbon and the two carbons connected by a double bond. This is really only part of the answer. In a double covalent bond, two pairs of electrons are shared between two atoms instead of one pair. Figure 10.3 shows how the electrons are shared in ethene.
Double bonds also hold molecules into rigid shapes, since there is no rotation or twisting possible at the double bond. Therefore, a general rule-of-thumb is that a molecule that does not possess a double or triple bond is more likely to be able to roll up or twist into different structures more easily. Figure 10.4 shows some common examples of double bonds.
Six-sided ring structures, like benzene, with alternating single and double bonds are called aromatic hydrocarbons. Many of these compounds have smells that most of us know. Oil of vanilla or vanillin (C 8 H 8 O 3 ), extracted from the fermented seed pods of the vanilla orchid, is shown in Figure 10.5 . Another common aromatic compound, cinnemaldehyde or oil of cinnamon (C 9 H 8 O), obtained from the steam distillation of cinnamon tree bark, is also shown.
When carbon forms a triple bond with another carbon, it is called an alkyne . Originally, triple-bonded carbon compounds were named after the simplest molecule, acetylene (C 2 H 2 ), which is HC≡CH. Dimethylacetylene is CH 3 C≡CCH 3 .
Alkynes are also known as acetylenes such as:
CH 3 C≡CH, propyne (methylacetylene)
CH 3 CH 2 C≡CH, 1-butyne (ethylacetylene)
Triple bonds involve the sharing of three pairs of electrons or six electrons.
Figure 10.6 shows some alkyne molecules.
When compounds contain double and triple bonds, the most common element interactions take place as addition reactions.
When a double-bonded hydrocarbon like ethene (CH 2 =CH 2 ) is added to another compound with single bonds, the elements of the new compound position themselves to either side of the double bond. The double bond is broken and single bonds are formed.
The addition of hydrogen chloride to ethene yields chloroethane:
CH 2 =CH 2 + HCl ⇒ H 3 −C−C−H 2 Cl ⇒ CH 3 CH 2 Cl
When diatomic chloride is added to ethene, then the result is 1,2-dichloro-ethane:
CH 2 =CH 2 + Cl 2 ⇒ ClH2−C−C−H 2 Cl ⇒ CH 2 CH 2 Cl 2
For triple-bonded hydrocarbons, the addition of bromine to ethyne would yield:
H−C≡C−H + 2 Br 2 ⇒ Br 2 H−C−C−HBr 2 ⇒ CH 2 Br 4
When carbons are arranged at the corners of a hexagon with a hydrogen bonded to each carbon and alternating double bonds between carbons, it is known as a ring structure or aromatic ring. The most basic ring structure is that of benzene (C 6 H 6 ).
Functional groups that are substituted for hydrogens around a ring bond to carbons and produce a variety of different molecules.
A homologous series of organic compounds are those that are very much alike structurally or react pretty much the same. The formula is the same for the compounds in the group with a CH 2 group added.
The alkanes are a homologous series of compounds. Their general formula expands each time by a −CH 2 group and is C n H 2 n +2 . Since they have only single covalent bonds, they are known as saturated compounds. Every carbon is bonded to four other atoms or groups.
Practice problems for these concepts can be found at – Organic Chemistry and Functional Groups Quiz
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