Chemistry lecture 30

2.13 SOURCES OF ALKANES AND CYCLOALKANES

As noted earlier, natural gas is especially rich in methane and also contains ethane and propane, along with smaller amounts of other low-molecular-weight alkanes. Natural gas is often found associated with petroleum deposits. Petroleum is a liquid mixture containing hundreds of substances, including approximately 150 hydrocarbons, roughly half of which are alkanes or cycloalkanes. Distillation of crude oil gives a number of fractions, which by custom are described by the names given in Figure 2.2. High-boiling fractions such as kerosene and gas oil find wide use as fuels for diesel engines and furnaces, and the nonvolatile residue can be processed to give lubricating oil, greases, petroleum jelly, paraffin wax, and asphalt.

Modern petroleum refining involves more than distillation, however, and includes two major additional operations:

1. Cracking. It is the more volatile, lower-molecular-weight hydrocarbons that are useful as automotive fuels and as a source of petrochemicals. Cracking increases the proportion of these hydrocarbons at the expense of higher molecular-weight ones by processes that involve the cleavage of carbon–carbon bonds induced by heat (thermal cracking) or with the aid of certain catalysts (catalytic cracking).

2. Reforming. The physical properties of the crude oil fractions known as light gasoline and naphtha (Figure 2.2) are appropriate for use as a motor fuel, but their ignition characteristics in high-compression automobile engines are poor and give rise to preignition, or “knocking.” Reforming converts the hydrocarbons in petroleum to aromatic hydrocarbons and highly branched alkanes, both of which show less tendency for knocking than unbranched alkanes and cycloalkanes.

The leaves and fruit of many plants bear a waxy coating made up of alkanes that prevents loss of water. In addition to being present in beeswax (see Problem 2.5), hentriacontane, CH₃(CH₂)₂₉CH₃, is a component of the wax of tobacco leaves.

Cyclopentane and cyclohexane are present in petroleum, but as a rule, unsubstituted cycloalkanes are rarely found in natural sources. Compounds that contain rings of various types, however, are quite abundant.

FUNCTIONS OF VITAMINS IN BIOSYSTEMS

Vitamins are the important food factors required in diet. Different vitamins are involved in different biochemical functions. They are required in very low concentration, the daily requirement of any vitamin for any individual is extremely smell. The daily dose of any vitamin is not a fixed quantity but varies according to size, age and rate of metabolism of the individual. Youngsters need higher quantity of vitamins then elders and their requirement increases when a person performs exercise. Growing children and pregnant mothers need more quantity of vitamins in their diet.

A lack of one or more vitamins leads to characteristic of vitamin deficiency is known as avitaminoses.

• Vitamin A is involved in vision, proper growth, reproduction and maintenance of epithelial cells.

• Vitamin A is essential in cholesterol synthesis.

• Vitamin D functions as a steroid hormone and regulates the plasma levels of calcium and phosphate.

• Vitamin E increases synthesis of heme and is also necessary for reproductive functions.

• Vitamin K is concerned with blood clotting process. It causes post translational modification of certain blood clotting factors.

• Vitamin C acts as coenzyme in hydroxylation of proline and lysine during collagen formation.

• It is also useful for bone formation and for preventing chronic diseases.

• Thiamine (Vitamin B₁) in its coenzyme form is involved in energy releasing reactions.

• The coenzymes of Riboflavin - FAD and FMN are involved in oxidation - reduction reactions.

• Pyridoxine involves in the synthesis of serotonin, histamine and niacin coenzymes from amino acids.

• Vitamin B₁₂ is used in isomerization of methyl malonyl COA to succinyl COA.

KNOWLEDGE FOR HEALTHY EATING

Welcome to The Nutrition Source, a Web site maintained by the Department of Nutrition at the Harvard School of Public Health.

Read more about healthy eating:

Healthy Eating Pyramid	Carbohydrates	Protein
	Vegetables/Fruits	Fats/Cholesterol

Aside from not smoking, the most important determinants of good health are what we eat and how active we are. The Nutrition Source is designed to get you started down the path toward the healthiest diet possible.

In the What Should You Eat section, you'll find eight key tips for eating right, plus our bottom line recommendations on carbohydrates, protein, fats, fiber, vegetables and fruits, calcium and milk, alcohol, and vitamins. You can also learn more about a food pyramid that's actually based on the latest science: the Healthy Eating Pyramid, created by the Department of Nutrition at Harvard School of Public Health. 

A lot of confusing information about nutrition gets batted about in the media and on the Web. The Nutrition Source will cut through all that confusion, providing clear tips for healthy eating and dispelling a few nutrition myths along the way.

Water is best to quench your thirst. Skip the sugary drinks, and go easy on the milk and juice.

1. Quit the sugar habit. The average can of sugar-sweetened soda or fruit punch provides about 150 calories. If you were to drink just one can of a sugar-sweetened soft drink every day, and not cut back on calories elsewhere, you could gain up to 15 pounds in a year. Cutting back on sugary drinks may help control your weight and may lower your risk of type 2 diabetes. See how much sugar is in your drink.

2. Go calorie-free naturally. “Diet” drinks with artificial sweeteners may condition our taste buds to crave super-sweet foods. Plain old water is the best calorie-free beverage—but if it’s just too plain, try adding a squeeze of lemon or lime or a splash of 100% fruit juice. Plain coffee and tea are also healthy calorie-free choices, in moderation. Read six ideas for low-sugar drinks.

3. If you don’t drink alcohol, there’s no need to start. Moderate alcohol consumption lowers the risk of heart disease and diabetes; it also slightly increases the risk of breast and colon cancer. For some people—especially pregnant women, people recovering from alcohol addiction, people with liver disease, and people taking one or more medications that interact with alcohol—the risks of drinking clearly outweigh the benefits.

4. Save sports drinks for athletes. Sports beverages are designed to give athletes carbs, electrolytes, and fluid during high-intensity workouts that last an hour or more. For sedentary folks, they’re just another source of sugary calories.

5. Pull the plug on energy drinks. These pricey concoctions have as much sugar as soft drinks, enough caffeine to raise your blood pressure, and an unpronounceable list of herbs and additives whose long-term health effects are unknown. No one needs them.

There are many options for what to drink, but without a doubt, water is the best choice: It's calorie-free, and it's as easy to find as the nearest tap.

Drinks that are loaded with sugar are the worst choice: They provide lots of calories and virtually no other nutrients. Drinking them routinely can lead to weight gain and increase the risk of type 2 diabetes.

Other drinks have pros and cons, but in moderation, can fit into a healthy diet:

·           Coffee and tea: These are calorie-free, as long as you don't load up on the sugar and cream. They are safe for most people and may even have some health benefits.

·           Artificially sweetened drinks: These have no calories—a plus—but their long-term effects on weight and health are unknown, so it's best to limit them, if you drink them at all.

·           100% fruit juice: Fruit juice has vitamins, but it is high in calories, so stick to no more than a small glass (four to six ounces) a day.

·           Milk: Milk is also high in calories, so there's no need to drink more than a glass or two of low fat or skim milk a day, and less is fine, if you get your calcium from other sources.

Alcohol: Alcohol is both a tonic and a poison, and the difference lies in the dose and the person drinking it; moderation is key.