The example compounds of ethene or ethylene and pentene are shown on the LEFT. Ethylene is the number one organic chemical synthesized in the U. What type of reaction is cracking? Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated: the original starting hydrocarbons are alkanes.
What is halogenation reaction? Halogenation is a reaction that occurs when one or more halogens are added to a substance. Halogens comprise the seventh column in the periodic table and include fluorine, chlorine, bromine, iodine, and astatine. The resulting product of a halogenation reaction is known as a halogenated compound. How do you identify an alkane?
Alkanes are identified because the carbon chain has only single bonds. Common alkanes include methane natural gas , propane heating and cooking fuel , butane lighter fluid and octane automobile fuel. Alkenes have at least one double bond and alkynes have at least one triple bond. What is the difference between alkanes and alkenes? Alkanes are hydrocarbons compounds containing only C and H that have single covalent bonds joining the carbon atoms.
We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive. Chlorinations and brominations are normally exothermic. Energy input in the form of heat or light is necessary to initiate these halogenations.
If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed. Halogenation reactions may be conducted in either the gaseous or liquid phase. In gas phase chlorinations the presence of oxygen a radical trap inhibits the reaction. In liquid phase halogenations radical initiators such as peroxides facilitate the reaction. The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms.
A chain reaction mechanism for the chlorination of methane has been described. Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.
To see an animated model of the bromination free radical chain reaction. When alkanes larger than ethane are halogenated, isomeric products are formed. Thus chlorination of propane gives both 1-chloropropane and 2-chloropropane as mono-chlorinated products.
Four constitutionally isomeric dichlorinated products are possible, and five constitutional isomers exist for the trichlorinated propanes.
Can you write structural formulas for the four dichlorinated isomers? The halogenation of propane discloses an interesting feature of these reactions. All the hydrogens in a complex alkane do not exhibit equal reactivity. For example, propane has eight hydrogens, six of them being structurally equivalent primary , and the other two being secondary. This particular resource used the following sources:.
Skip to main content. Organic Chemistry. Search for:. Reactions of Alkanes. Learning Objective Identify the general reactions of alkanes. Most of the benzene used commercially comes from petroleum. It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics.
Once widely used as an organic solvent, benzene is now known to have both short- and long-term toxic effects. The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed.
This results in a condition called aplastic anemia , in which there is a decrease in the numbers of both the red and white blood cells. How do the typical reactions of benzene differ from those of the alkenes? Briefly describe the bonding in benzene. Benzene is rather unreactive toward addition reactions compared to an alkene. Valence electrons are shared equally by all six carbon atoms that is, the electrons are delocalized.
The six electrons are shared equally by all six carbon atoms. Which compounds are aromatic? Five examples are shown below. In these structures, it is immaterial whether the single substituent is written at the top, side, or bottom of the ring: a hexagon is symmetrical, and therefore all positions are equivalent.
These compounds are named in the usual way with the group that replaces a hydrogen atom named as a substituent group: Cl as chloro, Br as bromo, I as iodo, NO 2 as nitro, and CH 3 CH 2 as ethyl. Although some compounds are referred to exclusively by IUPAC names, some are more frequently denoted by common names, as is indicated below. Some common aromatic hydrocarbons consist of fused benzene rings—rings that share a common side.
These compounds are called polycyclic aromatic hydrocarbons PAHs An aromatic hydrocarbon consisting of fused benzene rings sharing a common side. The three examples shown here are colorless, crystalline solids generally obtained from coal tar. Naphthalene has a pungent odor and is used in mothballs. Anthracene is used in the manufacture of certain dyes. Steroids, including cholesterol and the hormones, estrogen and testosterone, contain the phenanthrene structure.
The intense heating required for distilling coal tar results in the formation of PAHs. For many years, it has been known that workers in coal-tar refineries are susceptible to a type of skin cancer known as tar cancer. Investigations have shown that a number of PAHs are carcinogens. One of the most active carcinogenic compounds, benzopyrene, occurs in coal tar and has also been isolated from cigarette smoke, marijuana smoke, automobile exhaust gases, and charcoal-broiled steaks.
It is estimated that more than 1, t of benzopyrene are emitted into the air over the United States each year. Only a few milligrams of benzopyrene per kilogram of body weight are required to induce cancer in experimental animals. Benzo[a]pyrene is metabolized to produce biologically active compounds that can form physical adducts on DNA molecules.
These adducts can cause genetic mutations that cause cancer. Photo of cigarette smoke. Substances containing the benzene ring are common in both animals and plants, although they are more abundant in the latter. Plants can synthesize the benzene ring from carbon dioxide, water, and inorganic materials. Animals cannot synthesize it, but they are dependent on certain aromatic compounds for survival and therefore must obtain them from food.
Phenylalanine, tyrosine, and tryptophan essential amino acids and vitamins K, B 2 riboflavin , and B 9 folic acid all contain the benzene ring. Many important drugs, a few of which are shown in Table 8. So far we have studied only aromatic compounds with carbon-containing rings. However, many cyclic compounds have an element other than carbon atoms in the ring.
Organic ring structures that contain an atom other than carbon are called heterocyclic compounds. Within alkane structure there is free rotation about the carbon-to-carbon single bonds C—C.
In contrast, the structure of alkenes requires that the carbon atoms form a double bond. Double bonds between elements are created using p-orbital shells also called pi orbitals.
These orbital shells are shaped like dumbbells rather than the circular orbitals used in single bonds. This prevents the free rotation of the carbon atoms around the double bond, as it would cause the double bond to break during the rotation Figure 8. Thus, a single bond is analogous to two boards nailed together with one nail.
The boards are free to spin around the single nail. A double bond, on the other hand, is analogous to two boards nailed together with two nails. In the first case you can twist the boards, while in the second case you cannot twist them.
For molecules to create double bonds, electrons must share overlapping pi-orbitals between the two atoms. This requires the dumbbell-shaped pi-orbitals show on the left to remain in a fixed conformation during the double bond formation. This allows for the formation of electron orbitals that can be shared by both atoms shown on the right. Rotation around the double bond would cause the pi orbitals to be misaligned, breaking the double bond. Diagram provided from: JoJanderivative work — Vladsinger talk.
The fixed and rigid nature of the double bond creates the possibility of an additional chiral center, and thus, the potential for stereoisomers. New stereoisomers form if each of the carbons involved in the double bond has two different atoms or groups attached to it.
For example, look at the two chlorinated hydrocarbons in Figure 8. In the upper figure, the halogenated alkane is shown. Rotation around this carbon-carbon bond is possible and does not result in different isomer conformations. In the lower diagram, the halogenated alkene has restricted rotation around the double bond. Note also that each carbon involved in the double bond is also attached to two different atoms a hydrogen and a chlorine.
Thus, this molecules can form two stereoisomers: one that has the two chlorine atoms on the same side of the double bond, and the other where the chlorines reside on opposite sides of the double bond. For this section, we are not concerned with the naming that is also included in this video tutorial. The cis-trans naming system can be used to distinguish simple isomers, where each carbon of the double bond has a set of identical groups attached to it.
For example, in Figure 8. The cis and trans system, identifies whether identical groups are on the same side cis of the double bond or if they are on the opposite side trans of the double bond. For example, if the hydrogen atoms are on the opposite side of the double bond, the bond is said to be in the trans conformation. When the hydrogen groups are on the same side of the double bond, the bond is said to be in the cis conformation.
Notice that you could also say that if both of the chlorine groups are on the opposite side of the double bond, that the molecule is in the trans conformation or if they are on the same side of the double bond, that the molecule is in the cis conformation.
To determine whether a molecule is cis or trans , it is helpful to draw a dashed line down the center of the double bond and then circle the identical groups, as shown in figure 8.
Both of the molecules shown in Figure 8. Thus, the cis and trans designation, only defines the stereochemistry around the double bond, it does not change the overall identity of the molecule. However, cis and trans isomers often have different physical and chemical properties, due to the fixed nature of the bonds in space. Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups are unable to rotate about any of the ring carbon—carbon bonds.
Therefore, groups can be either on the same side of the ring cis or on opposite sides of the ring trans. For our purposes here, we represent all cycloalkanes as planar structures, and we indicate the positions of the groups, either above or below the plane of the ring. It relates to our consumption of dietary fats. Inappropriate or excessive consumption of dietary fats has been linked to many health disorders, such as diabetes and atherosclerosis, and coronary heart disease.
So what are the differences between saturated and unsaturated fats and what are trans fats and why are they such a health concern?
Photo from: TyMaHe. The most common form of dietary fats and the main constituent of body fat in humans and other animals are the triglycerides TAGs. TAGs, as shown in figure 8. In this section, we will focus on the structure of the long fatty acid tails, which can be composed of alkane or alkene structures.
Chapter 10 will focus more on the formation of the ester bonds. Notice that each triglyceride has three long chain fatty acids extending from the glycerol backbone. Each fatty acid can have different degrees of saturation and unsaturation. Structure adapted from: Wolfgang Schaefer. Fats that are fully saturated will only have fatty acids with long chain alkane tails. Saturated fats are common in the American diet and are found in red meat, dairy products like milk, cheese and butter, coconut oil, and are found in many baked goods.
Saturated fats are typically solids at room temperature. This is because the long chain alkanes can stack together having more intermolecular London dispersion forces. This gives saturated fats higher melting points and boiling points than the unsaturated fats found in many vegetable oils. Most of the unsaturated fats found in nature are in the cis -conformation, as shown in Figure 8. Note that the fatty acids shown in Figure 8. When the fatty acids from the TAG shown in Figure 8.
Thus, monounsaturated and polyunsaturated fats cannot stack together as easily and do not have as many intermolecular attractive forces when compared with saturated fats. As a result, they have lower melting points and boiling points and tend to be liquids at room temperature. It has been shown that the reduction or replacement of saturated fats with mono- and polyunsaturated fats in the diet, helps to reduce levels of the low-density-lipoprotein LDL form of cholesterol, which is a risk factor for coronary heart disease.
Trans-fats, on the other hand, contain double bonds that are in the trans conformation. Thus, the shape of the fatty acids is linear, similar to saturated fats.
Trans fats also have similar melting and boiling points when compared with saturated fats. However, unlike saturated fats, trans-fats are not commonly found in nature and have negative health impacts. Trans-fats occur mainly as a by-product in food processing mainly the hydrogenation process to create margarines and shortening or during cooking, especially deep fat frying.
In fact, many fast food establishments use trans fats in their deep fat frying process, as trans fats can be used many times before needing to be replaced. Consumption of trans fats raise LDL cholesterol levels in the body the bad cholesterol that is associated with coronary heart disease and tend to lower high density lipoprotein HDL cholesterol the good cholesterol within the body.
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