8.05 Carbon Chemistry

Carbon Atoms

Carbon atoms have the unique ability to bond with other carbon atoms in long chains, rings, and other formations. This property allows carbon to be the base of a variety of large molecules. Have you ever heard the term "carbon-based life-form"? All humans are carbon-based organisms, as are all animals and plants, because we are made up of many different carbon compounds. Life is actually based on carbon's ability to form diverse molecular structures. There is a branch of chemistry, called organic chemistry, dedicated to the study of compounds that contain carbon. Because organic compounds are the basis for living organisms and are also used for a variety of applications, such as fuels, medicines, agriculture, flavoring, and more, it is important to have a basic understanding of them.

Carbon

Carbon is a nonmetal that is found in nature both as a pure element and in various compounds. It is the 17th most abundant element, by mass, found on Earth, but it is one of the most important elements to humans because it is found in all living matter. A carbon atom has 4 valence electrons in its outermost energy level , and it has a strong tendency to share electrons to have a full octet in its valence. Carbon atoms often naturally bond with each other in chains, rings, plates, or networks, and they also bond readily with hydrogen, oxygen, nitrogen, sulfur, phosphorus, and the halogens. Carbon occurs in two pure forms, diamond and graphite. These two forms of carbon have very different properties due to the difference in how the carbon atoms are covalently bonded together. Diamonds are the hardest known material. This solid form of carbon is colorless with an extremely high density of 3.514 g/cm3. In a diamond, the carbon atoms are held together by all single bonds formed in a three-dimensional tetrahedral formation. This formation holds the carbon atoms together in a strong and compact structure that makes diamonds so strong and dense. Graphite is a soft, black, solid form of carbon that conducts electricity reasonably well and is easily broken. The "lead" in your pencil is actually graphite, so you can examine its properties for yourself. The carbon atoms in graphite are arranged in layers of thin hexagonal "plates." This configuration of carbon atoms is due to the fact that each carbon atom has one double bond and two single bonds, forming a trigonal planar shape of bonds around each carbon atom. The layers of carbon atoms in graphite are only held together by the weak London dispersion intermolecular force, which explains why graphite is so soft and easily broken.

Carbon Cycle

Carbon is an element found in all living things, the air, the water, rocks, soil, oceans, and fossil fuels. The cool thing about carbon is that all carbon is recycled as it moves from one step in the carbon cycle to the next. There is even carbon found in you. You're a part of this cycle, too, and that carbon could have come from anywhere in this cycle. Places in the cycle that you will find carbon: -atmosphere -fossil fuels -rocks -shells -soil -surface ocean -plants and trees -ocean life As carbon moves through this cycle there are many paths it may take. The pathways can be placed into two general categories, land based and water based. Land based part of the cycle: Plants and trees absorb/take carbon out of the air and use it to make their own food in a process called **photosynthesis**. Plants store carbon as they grow. Some of the carbon may be eaten by animals and stored in their bodies. Living things are made of cells and as these cells work they give off carbon as waste product. This is given off back into the air. When living things die they are broken down by **decomposers**. This leaves carbon that is stored in the ground in rocks and soil. Over long periods of time they are subject to great heat and pressure and turn into fossil fuels like coal and oil. We take fossil fuels out of the ground and burn them to create electricity or to make our cars run. Burning fossil fuels releases carbon dioxide back into the air. Sometimes plants and trees are burned to clear an area for farming or construction. This process can involve burning down large areas of forest. We call this **deforestation**. Burning the plants and trees releases the carbon stored in them into the atmosphere. Water based part of the carbon cycle: Carbon is held in sea water. As the air passes over the water some of the carbon will be taken up into the air and stored in the air. We call this process **diffusion**. Carbon in the water is also taken in by the sea plants and used to make their food in a process called **photosynthesis**. Carbon is stored in the plants and all the sea life that **consumes**, or eats, the plants. Some carbon is released back into sea water from living things through a process called **respiration**. During respiration cells work and give off carbon as a waste product that is released back into the water. When living things die they begin to be broken down and fall to the sea floor. They are then considered **sediment**. The carbon stored in sediment can become partially dissolved in the water or can be left in the Earth, where over millions of years, under great heat and pressure they will become fossil fuels. Certain sea creatures, like clam and oysters, can take the carbon from the water and use it to make shells. When the animal dies these shells fall to the sea floor and over time become rocks. Rocks will eventually break down and release the carbon back into the water. The carbon released deep in the ocean makes the deep waters high in carbon content. This carbon-rich water is brought to the surface by ocean **circulation**. Ocean circulation is a pattern of water flow that brings cold deep water to the surface. This carbon-rich water can release carbon into the atmosphere by the process of diffusion.

List the transitions that occurred in the cycle to recycle carbon from one compound to another.

Land: Water:

Categories of Organic Compounds

Organic compounds all contain carbon atoms. There are only a few carbon compounds, such as organic compounds like NaCO3 and the oxides of carbon (CO and CO2), that are not considered organic. Because of carbon's ability to form different combinations of single, double, and triple covalent bonds, the number of possible carbon compounds is virtually unlimited. There are more than 4 million naturally occurring and human-made organic compounds currently known. Carbon can fill its valence energy level by sharing a total of four pairs of electrons. This means that a given carbon atom can form 4 single bonds, 2 single bonds and 1 double bond, 2 double bonds, or 1 triple bond and 1 single bond.

Exploring Organic Compounds

There are many types of organic compounds and each type has its own unique properties. Hydrocarbons: Organic compounds that contain only carbon and hydrogen are called hydrocarbons. Methane, CH4, is the simplest hydrocarbon and is the main component of natural gas. Octane, a component of gasoline, is another hydrocarbon, with the formula C8H18. There is a large range of hydrocarbons because of the variety of ways that carbon atoms can bond together. Hydrocarbons can be chains, rings, or other shapes, and the carbons can bond together with single, double, or triple bonds. Propane (C3H8) is used as a heating fuel. This is an example of a saturated hydrocarbon because it is made up of only single bonds, meaning that it contains the maximum number of hydrogen atoms that the carbon chain can hold. Carbon atoms can bond in chains, but they can also form other shapes as the carbon chains branch apart. This compound, called isobutane or 2-methylpropane, is a small example of a branching hydrocarbon chain. Ethyne (C2H2), also known as acetylene, is used for underwater welding. Carbon chains that contain double or triple bonds are called unsaturated because they do not contain the maximum number of hydrogen atoms that they could hold. Benzene (C6H6) is a ring of six carbons that experience resonance between the bonds in the ring. This means that all of the carbon-carbon bonds in benzene are the same length and strength. Benzene is an industrial solvent used in many different fields. Naphthalene (C10H8) is commonly used in mothballs. Carbon atoms can bond to other atoms besides just hydrogen and carbon, which leads to various categories of organic compounds. Alcohols: Alcohols are organic compounds in which a hydroxyl group, an oxygen atom bonded to a hydrogen atom, is bonded to a carbon atom. Because of the polarity of the oxygen-hydrogen bond, smaller alcohol compounds are often soluble in water. There are many different types of alcohols; let's look at a few of the smaller alcohol molecules. Methanol (CH3OH) is commonly used for producing formaldehyde and acetic acid to be used industrially in the making of plastics. It is also added as an anti-icing agent to gasoline. Methanol is toxic and should never be ingested. Ethanol (C2H5OH) is the alcohol found in alcoholic beverages and is naturally produced by a process called fermentation. Isopropyl alcohol, or 2-propanol, is the rubbing alcohol that you can buy at the drugstore. It evaporates readily, which is why it has a pronounced cooling effect when placed on your skin. Isopropyl alcohol is very toxic and should never be ingested! Ethers: Ethers are organic compounds that have an oxygen atom bonded between two carbon atoms in the carbon chain. Ethers are not very soluble in water because they are unable to form strong hydrogen bonds with the water molecules. With the oxygen atom inside the carbon chain, the molecule has little polarity and the attraction between ether molecules is very weak. This means that it does not require much energy to separate ether molecules, giving them low boiling points. Diethyl ether, (C2H5)2O, was one of the first anesthetics. It helped to revolutionize medical surgeries in the mid-1800s. Amines: Amines are organic compounds that contain the amine group, a nitrogen atom bonded to one, two, or three carbons. Amines are typically less soluble in water than alcohols because the nitrogen-hydrogen bond is less polar than the oxygen-hydrogen bond. This lower polarity also means that amines will have a lower boiling point than alcohols of similar mass. Amines are often basic, because the nitrogen atoms in the carbon chain can readily accept a hydrogen ion from water. Many amines are known for their strong, offensive odors. This is why the common names of some amines include names such as putrescine and cadaverine, which are responsible in part for the odor of decaying flesh. A specific group of amines, called alkaloids, can be found in plants or marine organisms and have been found to have medical or other biological effects. One molecule that many people are familiar with, caffeine, is an example of an alkaloid. The Carbonyl Group: The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. Depending on where the carbonyl group appears on the organic compound and what else is bonded to it, the compound may fall into the category of ketone, aldehyde, amide, carboxylic acid, or ester. A ketone is an organic compound in which the carbonyl group is bonded to two other carbons. Acetone (CH3COCH3), often sold as fingernail polish remover, is a common example of a ketone. In an aldehyde, the carbonyl group is bonded to one carbon atom and one hydrogen atom or, in the special case of formaldehyde, to two hydrogen atoms. The carbonyl group is attached to a carbon atom on one side and a hydrogen atom on the other. Formaldehyde is an exception in that it contains a carbonyl group with only hydrogen atoms attached to the carbon of the carbonyl group. Aldehydes are often very fragrant; the pleasant scents of many flowers, lemons, cinnamon, and almonds are all due to the aldehydes. An amide is a carbonyl-containing compound in which the carbonyl group is bonded to a nitrogen atom. An amide called N,N-diethyl-m-toluamide, commercially known as DEET, is a common ingredient in mosquito repellents. This compound causes many insects, such as mosquitoes, to lose their sense of direction, hopefully protecting the wearer from being bitten. A carboxylic acid is an organic compound in which the carbonyl group is bonded to a hydroxyl group, oxygen bonded to hydrogen. This type of compound is able to donate a hydrogen ion from the hydroxyl group in an acid-base reaction. Acetic acid, a common acid found in vinegar, is an example of a simple carboxylic acid. In an ester, the carbonyl group is bonded to a carbon atom on one side and to an oxygen atom that is bonded to another carbon atom on the other side. Many simple ester compounds have notable fragrances and are used as artificial flavoring. Some artificial flavors and odors that are produced by esters include banana, orange, apple, and wintergreen. Aspirin, acetylsalicylic acid, is a compound that is both an ester and a carboxylic acid. Carbohydrates: Carbohydrates are the most abundant biomolecules on Earth made up of carbon, hydrogen, and oxygen atoms. Carbohydrates are often made up of aldehydes or ketones bonded together in more complex structures, and they have the empirical formula of CH2O. Carbohydrates are the structural elements in the cell walls of bacteria and plants and exoskeletons of invertebrates, and they serve as energy storage. Carbohydrates are the starting materials for many organic compounds like fats and amino acids. Sucrose is an example of a carbohydrate. Notice that it is made up of two carbon rings (one a pentagon and one a hexagon) that contain oxygen atoms and are linked together by an oxygen atom, and it also contains several hydroxyl groups. Polymers: Polymers are extremely long molecule chains that consist of repeated molecular units called monomers. Each monomer can be made up of anywhere from four to 100 atoms, and when chained together they can form polymers made up of hundreds or thousands of atoms total. These polymer molecules are still too small to be seen with the unaided eye, which is another reminder of just how small atoms really are! Human-made polymers, called synthetic polymers, are commonly known as plastics and have a wide variety of uses. Nylon, polystyrene, polypropylene, and PVC are examples of synthetic polymers. In the United States, synthetic polymers have surpassed steel as the most widely used material.