Topic 10 Organic Chemistry Ms. Kiely IB Chemistry (SL) Coral Gables Senior High School
-Alkanes: have low reactivity and undergo free radical substitution. -Alkenes: are more reactive than alkanes, since they are unsaturated, and undergo addition reactions. Bromine water can be used to distinguish between alkenes and alkanes. -Alcohols: undergo nucleophilic substitution reactions with acids (also called esterification or condensation) and some undergo oxidation reactions. -Halogenoalkanes: are more reactive than alkanes. They can undergo (nucleophilic) substitution reactions. A nucleophile is an electron-rich species containing a lone pair of electrons that it donates to an electron-deficient carbon -Polymers: addition polymers consist of a wide range of monomers and form the basis of the plastics industry. -Benzene: does not readily undergo addition reactions but does undergo electrophilic substitution reactions.
Benzene is an important organic chemical compound with the chemical formula C₆H₆. The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen atom attached to each. Benzene has no isomers! It is a cyclic structure, in which a framework of single bonds attaches each carbon to one on either side and to a hydrogen atom. This leaves one electron on each carbon atom. But instead of pairing up to form alternating double bonds, they are delocalized through all six carbon atoms! Delocalization of electrons in benzene produces a symmetrical cloud of electron charge above and below the plane of the ring.
When you draw benzene make sure not to draw it without a ring inside!
Benzene does not typically undergo addition reactions, only substitution reactions. Addition reactions, which would only lead to loss of the stable aromatic ring!!!!! Therefore, they are generally not favoured as the products would be of higher energy than the reactants. THEY WOULD BREAK THE RING! NO BUENO! Instead, substitution reactions, in which one or more of the hydrogen atoms is replaced by an incoming group, occur more readily as these lead to products in which the aromatic ring is conserved. The delocalized ring of electrons, which represents an area of electron density, is the site of reactivity- thus very important to maintain! Benzene molecules therefore mainly partake in substitution reactions!
Experimentally Deducing Organic Compounds We can use different kinds of structural analysis to determine the structure of unknown organic compounds that we are working with: The following instruments and techniques are helpful in determining the structure of organic compounds: 1. Mass spectronomy is used to determine relative atomic and molecular masses. 2. Degree of unsaturation/ihd provides a useful clue to the structure of a molecule once its formula is known. 3. Infrared spectroscopy is used to identify the bonds in a molecule. 4. Nuclear Magnetic Resonance (NMR) spectroscopy depends on a combination of nuclear physics and chemistry.
Degree of Unsaturation / IHD The degree of unsaturation or index of Hydrogen Deficiency (IHD) is a measure of how many molecules of H₂ would be needed in theory to convert the molecule to the corresponding saturated, non-cyclic molecule. The formula used to determine the IHD or degree of unsaturation of a molecule is the following: IHD = ½ [2n + 2 - p - s + r], with reference to the following subscripts CnHpOqNrXs When one of those atoms is not found in your formula, the value for the subscript is 0, zero. Example: Deduce the degree of unsaturation (the IHD) of bute, C₁₉H₂₀N₂O₂. n=19; p=20; q=2; r=2; s=0 ½ [2(19) + 2 - (20) - (0) + (2)] = ½ [40-20 - 2] = 9
Infrared (IR) spectroscopy We can gather information on what types of bonds are in a substance by use of infrared (IR) spectroscopy. When infrared light is shone on a substance, the bonds in that substance absorb the light. The absorption of this IR light causes particular bonds to either shake or bend. This shaking and bending is dependent on how much IR is absorbed- once we know how much IR was absorbed, we can discern which bonds are present. We can therefore use the infrared absorption spectra of a substance to determine what types of bonds are present.
Infrared (IR) spectroscopy Video: https://www.youtube.com/watch?v=mz-u7qpkz8y
Transmittance % Reference graph of types of bonds and their transmittance/absorption percentages.
Mass Spectronomy & NMR We will go over these when we review the Sample Paper 2 and 3 tests in class! Information on them can be found in Topic 11, section 11.2
Reactions with Alkanes Despite saturated compounds, such as the alkanes, being quite unreactive, there are still two types of reactions involving alkanes that we will cover: 1. Combustion reactions of alkanes 2. Substitution reactions of alkanes
1) Combustion; alkanes as fuels: alkanes are wisely used as fuels, whether for an engine or for household heating. The reason they are so effective in this case is because they are highly exothermic! Remember that a combustion reaction always requires a hydrocarbon and oxygen gas reacting to produce carbon dioxide and water: The large amount of heat released from the combustion reactions of alkanes is mainly because of the large amount of energy released in the formation of the C-O double bonds in carbon dioxide and the strong single O-H bonds in water.
2) Substitution reactions of alkanes; halogenation: As alkanes are saturated molecules, the main type of reaction that they can undergo is substitution. This occurs when another reactant, such as a halogen, takes the place of a hydrogen atom in the alkane. For example, methane gas reacts with chlorine gas to produce chloromethane gas and gaseous hydrogen chloride. The energy of UV light is needed to break the single covalent bond that holds Cl₂ together. This splits Cl₂ into two separate Cl atoms. The two neutral chlorine atoms, each with their 7 valence electrons, are referred to as free radicals: an uncharged molecule (typically highly reactive and short-lived) having an unpaired valence electron. Notice chloromethane gas is a halogenoalkane!
This substitution reaction, where chlorine, a halogen, takes the place of one of methane s hydrogen atoms, can be described in a sequence of steps called the reaction mechanism: 1) initiation, 2) propagation, and 3) termination. 1) Initiation
2) Propagation
3) Termination
Reactions with Alkenes -Unsaturated compounds that contain a carbon-carbon double bond. -Alkenes are more reactive than alkanes since the double bond is the site of reactivity of the molecule, and one of the bonds is relatively easily broken due to electrons populating the area of the bonds. -When the first bond of a double bond is broken, it creates two new bonding position on the carbon atoms, enabling alkenes to undergo addition reactions: 1. Addition of hydrogen 2. Addition of halogens 3. Addition of halogen halides (hydrogen bonded to a halogen, i.e. HCl) 4. Addition of water 5. Polymerization
Each of the alkene addition reactions result in the alkene forming into an alkane. One of the bonds from the double bond is broken; the two new sites on carbon are then used to synthesize the two reactants together: 1. Addition of hydrogen (known as hydrogenation) Propene + hydrogen gas Propane 2. Addition of halogens Propene + Bromine 1,2-dibromopropane
3. Addition of halogen halides (hydrogen bonded to a halogen, i.e. HCl) 4. Addition of water (known as hydration)
5. Polymerization of alkenes: Most of the reactions we have discussed in this lesson so far involve reactants and products of low molar mass. Some of the most important organic compounds that exist, however, are GIANT molecules called polymers. You see polymers every day; for instance, all plastics are polymers. A polymer is a large molecule formed by the repetition of covalently bonded small molecules. The small molecules that combine to form these polymers are called monomers.
Some polymers contain only one type of monomer, while others contains two or more types of monomers. An addition polymer forms when unsaturated monomers (i.e. alkenes) react to form a polymer. The reaction that joins monomers to form a polymer is called polymerization, and it usually requires a catalyst.
Ethene Polyethene Note that the letter n on the reactant side of the equation refers to the number of monomers (in this case, ethene) that combine to form the polymer that is on the product side (in this case, polyethene). The letter n on the product side of the equation (next to polyethene in this case) indicates the number of repeating units in the polymer. Parentheses are used to identify the repeating unit; in this case, CH2.
Polyethene, which is chemically resistant and easy to clean, is an important industrial product. It is used to make plastic bottles, containers, toys, and other everyday plastic materials. The physical properties of polyethene can be controlled by shortening and lengthening the carbon chains of the molecules. For example, shortening the chains creates a more moldable, malleable plastic, whereas lengthening the chains causes polyethene to be harder and more rigid.
Propene Polypropylene Similarly, propene polymerizes to form poly(propene), often called polypropylene. This is a stiffer polymer than polyethene. It is used extensively in utensils and beverage containers.
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