The definitions are given by how many carbon atoms attached to that carbon. And we also have quarternary for carbon with four other carbon atoms on it, but rarely used. A carbon price is a fee on each unit of carbon dioxide (CO2) or other greenhouse gas emissions released into the atmosphere. Mezzo artist paint & brush racks. There are two primary methods of pricing carbon-carbon taxes and cap-and-trade programs. Carbon taxes would directly establish a price on carbon in dollars per ton of emissions.
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Here at StudyOrgo, we frequently get questions about topics in organic chemistry that are usually quickly covered, poorly described or expected that you know from previous courses. These concepts are really important to understanding the more complex topics to come. In this article, we will cover the concepts of stereochemistry descriptions using bold and wedged bonds. This is just a preview of the detailed topics and materials available with your membership to StudyOrgo.com. Sign up today!
Substitution reactions involve the attack by an electron-rich element, referred to as the nucleophile, on an electron-poor atom, referred to as the electrophile. As the reaction name suggests, we are substituting the nucleophile for another group on the electrophile atom, which is referred to as the leaving group. The generic reaction looks like this.
In Substitution reactions, there are two mechanisms that will be observed. An Sn2 and Sn1 reaction mechanism.
Sn2 reactions are bimolecular in rate of reaction and have a concerted mechanism. The process involves simultaneous bond formation by the nucleophile and bond cleavage by the leaving group. The transition state looks like this. Because the reaction is concerted, Sn2 mechanisms will always lead to an inversion of stereochemistry! For reactivity using an Sn2 mechanism, primary >> secondary >> tertiary carbon centers.
On the other hand, Sn1 reactions are unimolecular in rate of reaction and have a step-wise mechanism. This process first involves bond cleavage by the LG to generate a carbocation intermediate. The stability of carbocation formation will determine if Sn1 or Sn2 reactions occur. In the second step, the electronegative nucleophile attacks the carbocation to form the product. The steps look like this. Because the nucleophile can attach either side of the carbocation, which adopts an sp2-hybridized orbital with a trigonal planar geometry, an equal amount of inversion and retention is seen, referred to as a racemic mixture. For reactivity using an Sn1 mechanism, tertiary >> secondary >>> primary carbon centers.
The strength of nucleophiles used help to determine the reaction mechanism. Strong bases will almost always proceed to Sn2 mechanism. Weak nucleophiles will generally proceed to Sn1 mechanism when a stable carbocation is present. Below is a list of nucleophile trends in order of nucleophile strength.
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POOLS,FLUXES AND A WORD ABOUT UNITS
In order to understand how carbon is cycled and how atmosphericCO2 will change in the future, scientists must carefully study theplaces in which carbon is stored (pools), how long it resides there,and processes that transfer it from one pool to another (fluxes). Collectively, all of the major pools and fluxes of carbon on Earthcomprise what we refer to as the global carbon cycle.
As you might imagine, the actual global carbon cycle is immenselycomplex. It includes every plant, animal and microbe, everyphotosynthesizing leaf and fallen tree, every ocean, lake, pond andpuddle, every soil, sediment and carbonate rock, every breath of freshair, volcanic eruption and bubble rising to the surface of a swamp,among much, much else. Because we can't deal with that level ofcomplexity, scientists often describe the carbon cycle by lumpingsimilar objects or environments into simpler groups (forest, grassland,atmosphere, ocean) and focusing only on the processes that are mostimportant at the global scale (see GlobalCarbon Cycle Diagram). As you mightimagine, part of the trick is understanding just what those processesare.
The following section is a brief overview of some of the importantpools and fluxes in the global carbon cycle (and note that, in ourdiscussion, we will use the terms pool, stock and reservoirinterchangeably). But first, it’s worth taking a moment toconsider the numbers and units scientists often deal with. Because the quantities of carbon in the Earth’s major carbon pools canbe quite large, it is inconvenient to use familiar units such as poundsor kilograms. Instead, we use other units that are better suitedfor expressing large numbers. For example, a Petagram of carbon (Pg),also known as a Gigaton (Gt), is equal to 10^15 grams or one billiontonnes. A tonne, also known as a metric ton, is equal to onethousand kilograms (1,000 kg). Because one kilogram is equal to2.205 pounds, one metric tonne is the same as 2205 pounds. Takingthis further, we can see that one Petagram is equal to just about2,200,000,000,000 (or 2.2 trillion) pounds! Expressing this as 1Pg is much simpler than working with that many zeros. Now we willconsider carbon stored on Earth in four main reservoirs.
Depending on our goals, the Earth’s carbon pools can be grouped intoany number of different categories. Here, we will consider fourcategories that have the greatest relevance to the overall carboncycle. Keep in mind that any of these pools could be furtherdivided into a number of subcategories, as we will occasionallydiscuss.
The Earth’s Crust: Thelargest amount of carbon on Earth is stored insedimentary rocks within the planet’s crust. These are rocksproduced either by the hardening of mud (containing organic matter)into shale over geological time, or by the collection of calciumcarbonate particles, from the shells and skeletons of marine organisms,into limestone and other carbon-containing sedimentary rocks. Together all sedimentary rocks on Earth store 100,000,000 PgC.Recalling that 1 Pg is over two trillion pounds, this is clearly alarge mass of carbon! Another 4,000 PgC is stored in the Earth’scrust as hydrocarbons formed over millions of years from ancient livingorganisms under intense temperature and pressure. Thesehydrocarbons are commonly known as fossil fuels.
Oceans: TheEarth’s oceans contain 38,000 PgC, most of which isin the form of dissolved inorganic carbon stored at great depths whereit resides for long periods of time. A much smaller amount ofcarbon, approximately 1,000 Pg, is located near the oceansurface. This carbon is exchanged rapidly with the atmospherethrough both physical processes, such as CO2 gas dissolving into thewater, and biological processes, such as the growth, death and decay ofplankton. Although most of this surface carbon cycles rapidly,some of it can also be transferred by sinking to the deep ocean poolwhere it can be stored for a much longer time.
Atmosphere: Theatmosphere contains approximately 750 PgC, most ofwhich is in the form of CO2, with much smaller amounts of methane (CH4and various other compounds). Although this is considerably lesscarbon than that contained in the oceans or crust, carbon in theatmosphere is of vital importance because of its influence on thegreenhouse effect and climate. The relatively small size of theatmospheric C pool also makes it more sensitive to disruptions causedby and increase in sources or sinks of C from the Earth’s otherpools. In fact, the present-day value of 750 PgC is substantiallyhigher than that which occurred before the onset of fossil fuelcombustion and deforestation. Before these activities began, theatmosphere contained approximately 560 PgC and this value is believedto be the normal upper limit for the Earth under naturalconditions. In the context of global pools and fluxes, theincrease that has occurred in the past several centuries is the resultof C fluxes to the atmosphere from the crust (fossil fuels) andterrestrial ecosystems (via deforestation and other forms of landclearing).
Terrestrial Ecosystems:Terrestrial ecosystems contain carbon in theform of plants, animals, soils and microorganisms (bacteria andfungi). Of these, plants and soils are by far the largest and,when dealing with the entire globe, the smaller pools are oftenignored. Unlike the Earth’s crust and oceans, most of the carbonin terrestrial ecosystems exists in organic forms. In thiscontext, the term “organic” refers to compounds that were produced byliving things, including leaves, wood, roots, dead plant material andthe brown organic matter in soils (which is the decomposed remains offormerly living tissues).
Plants exchange carbon with the atmosphere relatively rapidly throughphotosynthesis, in which CO2 is absorbed and converted into new planttissues, and respiration, where some fraction of the previouslycaptured CO2 is released back to the atmosphere as a product ofmetabolism. Of the various kinds of tissues produced by plants,woody stems such as those produced by trees have the greatest abilityto store large amounts of carbon. Wood is dense and trees can belarge. Collectively, the Earth’s plants store approximately 560PgC, with the wood in trees being the largest fraction.
The total amount of carbon in the world’s soils is estimated to be 1500PgC. Measuring soil carbon can be challenging, but a few basicassumptions can make estimating it much easier. First, the mostprevalent form of carbon in the soil is organic carbon derived fromdead plant materials and microorganisms. Second, as soil depthincreases the abundance of organic carbon decreases. Standardsoil measurements are typically only taken to 1m in depth. Inmost case, this captures the dominant fraction of carbon in soils,although some environments have very deep soils where this rule doesn’tapply. Most of the carbon in soils enters in the form of deadplant matter that is broken down by microorganisms during decay. The decay process also released carbon back to the atmosphere becausethe metabolism of these microorganisms eventually breaks most of theorganic matter all the way down to CO2.
The movement of any material from one place to another is called a fluxand we typically think of a carbon flux as a transfer of carbon fromone pool to another. Fluxes are usually expressed as a rate withunits of an amount of some substance being transferred over a certainperiod of time (e.g. g cm-2 s-1 or kg km2 yr-1). For example, theflow of water in a river can be thought of as a flux that transferswater from the land to the sea and can be measured in gallons perminute or cubic kilometers per year.
A single carbon pool can often have several fluxes both adding andremoving carbon simultaneously. For example, the atmosphere hasinflows from decomposition (CO2 released by the breakdown of organicmatter), forest fires and fossil fuel combustion and outflows fromplant growth and uptake by the oceans. The size of various fluxescan vary widely. In the previous section, we briefly discussed afew of the fluxes into and out of various global C pools. Here,we will pay more careful attention to some of the more important Cfluxes.
Photosynthesis: Duringphotosynthesis, plants use energy from sunlightto combine CO2 from the atmosphere with water from the soil to createcarbohydrates (notice that the two parts of the word, carbo- and–hydrate, signify carbon and water). In this way, CO2 is removedfrom the atmosphere and stored in the structure of plants. Virtually all of the organic matter on Earth was initially formedthrough this process. Because some plants can live to be tens,hundreds or sometimes even thousands of years old (in the case of thelongest-living trees), carbon may be stored, or sequestered, forrelatively long periods of time. When plants die, their tissuesremain for a wide range of time periods. Tissues such as leaves,which have a high quality for decomposer organisms, tend to decayquickly, while more resistant structures, such as wood can persist muchlonger. Current estimates suggest photosynthesis removes 120PgC/year from the atmosphere and about 610 PgC is stored in plants atany given time.
Plant Respiration:Plants also release CO2 back to the atmospherethrough the process of respiration (the plant equivalent ofexhaling). Respiration occurs as plant cells use carbohydrates,made during photosynthesis, for energy. Plant respirationrepresents approximately half (60 PgC/year) of the CO2 that is returnedto the atmosphere in the terrestrial portion of the carbon cycle.
Litterfall: Inaddition to the death of whole plants, livingplants also shed some portion of their leaves, roots and branches eachyear. Because all parts of the plant are made up of carbon, theloss of these parts to the ground is a transfer of carbon (a flux) fromthe plant to the soil. Dead plant material is often referred toas litter (leaf litter, branch litter, etc.) and once on the ground,all forms of litter will begin the process of decomposition.
Soil Respiration: Therelease of CO2 through respiration is not uniqueto plants, but is something all organisms do. When dead organicmatter is broken down ordecomposed (consumed by bacteria and fungi), CO2 is released into theatmosphere at an average rate of about 60 PgC/year globally. Because it can take years for a plant to decompose (or decades in thecase of large trees), carbon is temporarily stored in the organicmatter of soil.
Ocean—Atmosphere exchange:Inorganic carbon is absorbed and released atthe interface of the oceans’ surface and surrounding air, through theprocess of diffusion. It may not seem obvious that gasses can bedissolved into, or released from water, but this is what leads to theformation of bubbles that appear in a glass of water left to sit for along enough period of time. The air contained in those bubblesincludes CO2 and this same process is the first step in the uptake ofcarbon by oceans. Once in a dissolved form, CO2 goes on to reactwith water in what are known as the carbonate reactions. Theseare relatively simple chemical reactions in which H2O and CO2 join toform H2CO3 (also known as carbonic acid, the anion of which, CO3, iscalled carbonate). The formation of carbonate in seawater allowsoceans to take up and store a much larger amount of carbon than wouldbe possible if dissolved CO2 remained in that form. Carbonate isalso important to a vast number of marine organisms that use thismineral form of carbon to build shells.
Carbon is also cycled through the ocean by the biological processes ofphotosynthesis, respiration, and decomposition of aquatic plants. In contrast with terrestrial vegetation is the speed at which marineorganisms decompose. Because ocean plants don’t have large, woodytrunks that take years to breakdown, the process happens much morequickly in oceans than on land—often in a matter of days. Forthis reason, very little carbon is stored in the ocean throughbiological processes. The total amount of carbon uptake (92 Pg C)and carbon loss (90 PgC) from the ocean is dependent on the balance oforganic and inorganic processes.
Fossil fuel combustion andland cover change: The carbon fluxesdiscussed thus far involve natural processes that have helped regulatethe carbon cycle and atmospheric CO2 levels for millions ofyears. However, the modern-day carbon cycle also includes severalimportant fluxes that stem from human activities. The mostimportant of these is combustion of fossil fuels: coal, oil and naturalgas. These materials contain carbon that was captured by livingorganisms over periods of millions of years and has been stored invarious places within the Earth's crust (see accompanying textbox). However, since the onset of the industrial revolution,these fuels have been mined and combusted at increasing rates and haveserved as a primary source of the energy that drives modern industrialhuman civilization. Because the main byproduct of fossil fuelcombustion is CO2, these activities can be viewed in geological termsas a new and relatively rapid flux to the atmosphere of large amountsof carbon. At present, fossil fuel combustion represents a fluxto the atmosphere of approximately 6-8 PgC/year.
Another human activity that has caused a flux of carbon to theatmosphere is land cover change, largely in the form ofdeforestation. With the expansion of the human population andgrowth of human settlements, a considerable amount of the Earth's landsurface has been converted from native ecosystems to farms and urbanareas. Native forests in many areas have been cleared for timberor burned for conversion to farms and grasslands. Because forestsand other native ecosystems generally contain more carbon (in bothplant tissues and soils) than the cover types they have been replacedwith, these changes have resulted in a net flux to the atmosphere ofabout 1.5 PgC/year. In some areas, regrowth of forests from pastland clearing activities can represent a sink of carbon (as in the caseof forest growth following farm abandonment in eastern North America),but the net effect of all human-induced land cover conversions globallyrepresents a source to the atmosphere.
Geological Processes:Geological processes represent an importantcontrol on the Earth's carbon cycle over time scales of hundreds ofmillions of years. A thorough discussion of the geological carboncycle is beyond the scope of this introduction, but the processesinvolved include the formation of sedimentary rocks and their recyclingvia plate tectonics, weathering and volcanic eruptions.
To take a slightly closer look, rocks on land are broken down by theatmosphere, rain, and groundwater into small particles and dissolvedmaterials, a process known as weathering. These materials arecombined with plant and soil particles that result from decompositionand surface erosion and are later carried to the ocean where the largerparticles are deposited near shore. Slowly, these sedimentsaccumulate, burying older sediments below. The layering ofsediment causes pressure to build and eventually becomes so great thatdeeper sediments are turned into rock, such as shale. Within theocean water itself, dissolved materials mix with seawater and are usedby marine life to make calcium carbonate (CaCO3) skeletons andshells. When these organisms die, their skeletons and shells sinkto the bottom of the ocean. In shallow waters (less than 4km) thecarbonate collects and eventually forms another type of sedimentaryrock called limestone.
Collectively, these processes convert carbon that was initiallycontained in living organisms into sedimentary rocks within the Earth'scrust. Once there, these materials continue to be moved andtransformed through the process of plate tectonics, uplift of rockscontained in the lighter plates and melting of rocks in the heavierplates as they are pushed deep under the surface. These meltedmaterials can eventually result in emission of gaseous carbon back tothe atmosphere through volcanic eruptions, thereby completing thecycle. Although the recycling of carbon through sedimentary rocksis vital to our planet's long-term ability to sustain life, thegeological cycle moves so slowly that these fluxes are small on anannual basis and have little effect on a human time-scale.